diff --git a/src/Integrations/Divi/Design/ColorBook.php b/src/Integrations/Divi/Design/ColorBook.php
new file mode 100644
index 0000000..a3c5238
--- /dev/null
+++ b/src/Integrations/Divi/Design/ColorBook.php
@@ -0,0 +1,265 @@
+<?php
+
+declare(strict_types=1);
+
+namespace LIT\MCP\Integrations\Divi\Design;
+
+use Closure;
+use DateTimeImmutable;
+use DateTimeZone;
+use InvalidArgumentException;
+use LIT\MCP\Abilities\Rev;
+use LIT\MCP\Integrations\Divi\Compose\VariableRef;
+
+defined('ABSPATH') || exit;
+
+/**
+ * The global colors in Divi's format (GlobalData::get_global_colors(), formats §1): id => {color, folder, label,
+ * lastUpdated, status, usedInPosts}. Takes the shape of get_global_colors() and of a snapshot alike (the same
+ * entries), fills missing keys and keeps only these six. The five customizer colors carry lastUpdated "" – Divi
+ * creates it fresh on every read, so design_rev and snapshots stay stable (ruling V3). Immutable – with() returns a
+ * new book. rev() is the design_rev of the area "colors". Pure PHP.
+ */
+final class ColorBook
+{
+    /** Customizer color ID => option name in et_divi (GlobalData::$customizer_colors). */
+    public const CUSTOMIZER_IDS = [
+        'gcid-primary-color' => 'accent_color',
+        'gcid-secondary-color' => 'secondary_accent_color',
+        'gcid-heading-color' => 'header_color',
+        'gcid-body-color' => 'font_color',
+        'gcid-link-color' => 'link_color',
+    ];
+
+    /** A color that refers to another one resolves at most this many references deep (Divi: depth 10). */
+    private const MAX_DEPTH = 10;
+
+    private const ALPHABET = '0123456789abcdefghijklmnopqrstuvwxyz';
+
+    /** @param array<string, array{color: string, folder: string, label: string, lastUpdated: string,
+     *     status: string, usedInPosts: list<mixed>}> $data */
+    private function __construct(private array $data)
+    {
+    }
+
+    /** @param array<array-key, mixed>|object $raw get_global_colors() or a snapshot – entries may be objects. */
+    public static function fromDivi(array|object $raw): self
+    {
+        $data = [];
+        foreach (self::plain($raw) as $id => $entry) {
+            if (is_array($entry)) {
+                $data[(string) $id] = self::normalize((string) $id, $entry);
+            }
+        }
+
+        return new self($data);
+    }
+
+    /** @return array<string, array{color: string, folder: string, label: string, lastUpdated: string,
+     *     status: string, usedInPosts: list<mixed>}> */
+    public function data(): array
+    {
+        return $this->data;
+    }
+
+    /** Hash of the canonical JSON (keys sorted recursively), so the order of the entries does not matter. */
+    public function rev(): string
+    {
+        // phpcs:ignore WordPress.WP.AlternativeFunctions.json_encode_json_encode -- pure PHP (spec §4.4).
+        $json = (string) json_encode(self::sorted($this->data), JSON_UNESCAPED_UNICODE | JSON_UNESCAPED_SLASHES);
+
+        return Rev::of('colors' . "\0" . $json);
+    }
+
+    /** @return array{color: string, folder: string, label: string, lastUpdated: string, status: string,
+     *     usedInPosts: list<mixed>}|null */
+    public function entry(string $id): ?array
+    {
+        return $this->data[$id] ?? null;
+    }
+
+    /** The label of $id, else $id itself. */
+    public function label(string $id): string
+    {
+        $label = $this->data[$id]['label'] ?? '';
+
+        return $label === '' ? $id : $label;
+    }
+
+    public function isCustomizer(string $id): bool
+    {
+        return isset(self::CUSTOMIZER_IDS[$id]) || ($this->data[$id]['folder'] ?? '') === 'customizer';
+    }
+
+    /**
+     * The ID of the color $needle names: an ID, else an alias (primary, secondary, heading, body, link), else the
+     * option name of a customizer color (accent_color …), else the one color of that label (case-insensitive).
+     *
+     * @throws InvalidArgumentException When several colors share the label (ruling S13a).
+     */
+    public function find(string $needle): ?string
+    {
+        if (isset($this->data[$needle])) {
+            return $needle;
+        }
+
+        $key = mb_strtolower(trim($needle));
+        $alias = DesignSystem::COLOR_ALIASES[$key] ?? array_search($key, self::CUSTOMIZER_IDS, true);
+        if (is_string($alias) && isset($this->data[$alias])) {
+            return $alias;
+        }
+
+        $ids = [];
+        foreach ($this->data as $id => $entry) {
+            if ($key !== '' && mb_strtolower(trim($entry['label'])) === $key) {
+                $ids[] = $id;
+            }
+        }
+        if (count($ids) > 1) {
+            throw new InvalidArgumentException(sprintf(
+                'Name der globalen Farbe "%s" ist mehrdeutig; bitte die ID angeben: %s.',
+                trim($needle),
+                implode(', ', $ids)
+            ));
+        }
+
+        return $ids[0] ?? null;
+    }
+
+    /** @param array<string, mixed> $entry Added or replaced; missing keys are filled like fromDivi() does. */
+    public function with(string $id, array $entry): self
+    {
+        $data = $this->data;
+        $data[$id] = self::normalize($id, $entry);
+
+        return new self($data);
+    }
+
+    /** @return list<string> IDs of the colors whose value is a reference to $id (relative colors). */
+    public function referencing(string $id): array
+    {
+        return array_keys(array_filter(
+            $this->data,
+            static fn (array $entry): bool => self::targetOf($entry['color']) === $id
+        ));
+    }
+
+    /**
+     * The first chain of references that loops or is deeper than Divi resolves (more than 10 references), as the
+     * IDs from its start to its end (a loop ends with the ID it began to repeat); null when there is none.
+     *
+     * @return list<string>|null
+     */
+    public function cycle(): ?array
+    {
+        foreach (array_keys($this->data) as $start) {
+            $chain = [$start];
+            $id = $start;
+            while (isset($this->data[$id]) && ($next = self::targetOf($this->data[$id]['color'])) !== null) {
+                $chain[] = $next;
+                if (in_array($next, array_slice($chain, 0, -1), true) || count($chain) - 1 > self::MAX_DEPTH) {
+                    return $chain;
+                }
+                $id = $next;
+            }
+        }
+
+        return null;
+    }
+
+    /**
+     * New and changed entries compared to $before – what the normal write path hands to Divi (ruling V4). The
+     * customizer colors' lastUpdated is "" in both books and never counts.
+     *
+     * @return array<string, array{color: string, folder: string, label: string, lastUpdated: string,
+     *     status: string, usedInPosts: list<mixed>}>
+     */
+    public function changedSince(self $before): array
+    {
+        return array_filter(
+            $this->data,
+            static fn (array $entry, string $id): bool => $before->entry($id) !== $entry,
+            ARRAY_FILTER_USE_BOTH
+        );
+    }
+
+    /** A new color ID like the Visual Builder's: "gcid-" and 10 characters of [0-9a-z] (formats §1). */
+    public static function newId(?Closure $random = null): string
+    {
+        $random ??= static function (int $length): string {
+            $id = '';
+            for ($i = 0; $i < $length; ++$i) {
+                $id .= self::ALPHABET[random_int(0, 35)];
+            }
+
+            return $id;
+        };
+
+        return 'gcid-' . $random(10);
+    }
+
+    /** lastUpdated of a color written now: Y-m-d\TH:i:s.v\Z in UTC (formats §1). */
+    public static function now(): string
+    {
+        return (new DateTimeImmutable('now', new DateTimeZone('UTC')))->format('Y-m-d\TH:i:s.v\Z');
+    }
+
+    /**
+     * @param array<array-key, mixed> $entry
+     * @return array{color: string, folder: string, label: string, lastUpdated: string, status: string,
+     *     usedInPosts: list<mixed>}
+     */
+    private static function normalize(string $id, array $entry): array
+    {
+        $text = static fn (string $key, string $default): string => is_scalar($entry[$key] ?? null)
+            ? (string) $entry[$key]
+            : $default;
+        $folder = $text('folder', '');
+
+        return [
+            'color' => $text('color', ''),
+            'folder' => $folder,
+            'label' => $text('label', ''),
+            'lastUpdated' => isset(self::CUSTOMIZER_IDS[$id]) || $folder === 'customizer'
+                ? ''
+                : $text('lastUpdated', ''),
+            'status' => $text('status', 'active'),
+            'usedInPosts' => is_array($entry['usedInPosts'] ?? null) ? array_values($entry['usedInPosts']) : [],
+        ];
+    }
+
+    /** The color ID $color refers to when it is exactly one $variable(…)$ color reference, else null. */
+    private static function targetOf(string $color): ?string
+    {
+        if (VariableRef::typeOf($color) !== 'color') {
+            return null;
+        }
+        // typeOf() proved a whole reference: strip marker and end like Divi does, the JSON object is in between.
+        $payload = json_decode(substr($color, strlen(VariableRef::MARKER), -strlen(VariableRef::END)), true);
+        $name = is_array($payload) ? ($payload['value']['name'] ?? null) : null;
+
+        return is_string($name) ? $name : null;
+    }
+
+    private static function sorted(mixed $value): mixed
+    {
+        if (! is_array($value)) {
+            return $value;
+        }
+        $value = array_map([self::class, 'sorted'], $value);
+        if (! array_is_list($value)) {
+            ksort($value);
+        }
+
+        return $value;
+    }
+
+    private static function plain(mixed $value): mixed
+    {
+        if (is_object($value)) {
+            $value = get_object_vars($value);
+        }
+
+        return is_array($value) ? array_map([self::class, 'plain'], $value) : $value;
+    }
+}
diff --git a/tests/Unit/Integrations/Divi/Design/ColorBookTest.php b/tests/Unit/Integrations/Divi/Design/ColorBookTest.php
new file mode 100644
index 0000000..d7844fd
--- /dev/null
+++ b/tests/Unit/Integrations/Divi/Design/ColorBookTest.php
@@ -0,0 +1,244 @@
+<?php
+
+declare(strict_types=1);
+
+namespace LIT\MCP\Tests\Unit\Integrations\Divi\Design;
+
+use InvalidArgumentException;
+use LIT\MCP\Integrations\Divi\Compose\VariableRef;
+use LIT\MCP\Integrations\Divi\Design\ColorBook;
+use PHPUnit\Framework\TestCase;
+use stdClass;
+
+final class ColorBookTest extends TestCase
+{
+    /** @return array<string, array<string, mixed>> The shape of GlobalData::get_global_colors() (formats §1). */
+    public static function stored(string $customizerStamp = '2026-10-19T08:00:00.000Z'): array
+    {
+        $entry = static fn (
+            string $color,
+            string $label,
+            string $folder = '',
+            string $stamp = '2026-09-27T22:21:39.000Z',
+            string $status = 'active'
+        ): array => [
+            'color' => $color, 'folder' => $folder, 'label' => $label, 'lastUpdated' => $stamp,
+            'status' => $status, 'usedInPosts' => [],
+        ];
+
+        return [
+            'gcid-primary-color' => $entry('#2ea3f2', 'Primärfarbe', 'customizer', $customizerStamp),
+            'gcid-secondary-color' => $entry('#2ea3f2', 'Sekundärfarbe', 'customizer', $customizerStamp),
+            'gcid-q9swzy97df' => $entry('#5b3a29', 'Holz dunkel'),
+            'gcid-989stx15zc' => $entry(self::ref('gcid-q9swzy97df', ['lightness' => 30]), 'Holz hell'),
+            'gcid-aaaaaaaaaa' => $entry('#111111', 'Doppelt'),
+            'gcid-bbbbbbbbbb' => $entry('#222222', 'doppelt', '', '2026-09-27T22:21:39.000Z', 'inactive'),
+        ];
+    }
+
+    /** @param array<string, int|float> $settings */
+    public static function ref(string $id, array $settings = []): string
+    {
+        return VariableRef::serialize('color', $id, $settings);
+    }
+
+    public function testMissingKeysAreFilledAndOnlyTheSixKeysStay(): void
+    {
+        $book = ColorBook::fromDivi([
+            'gcid-x1' => ['color' => '#fff', 'extra' => 'dropped'],
+            'gcid-x2' => (object) ['color' => '#000', 'label' => 'Schwarz', 'usedInPosts' => [4 => 12]],
+            'gcid-x3' => 'not an entry',
+        ]);
+
+        self::assertSame(['gcid-x1', 'gcid-x2'], array_keys($book->data()));
+        self::assertSame([
+            'color' => '#fff', 'folder' => '', 'label' => '', 'lastUpdated' => '', 'status' => 'active',
+            'usedInPosts' => [],
+        ], $book->entry('gcid-x1'));
+        self::assertSame([12], $book->entry('gcid-x2')['usedInPosts']);
+        self::assertSame('Schwarz', $book->label('gcid-x2'));
+        self::assertSame('gcid-x1', $book->label('gcid-x1'), 'a color without a label shows its ID');
+        self::assertNull($book->entry('gcid-nope'));
+    }
+
+    public function testObjectInputIsAcceptedLikeAnArray(): void
+    {
+        $raw = new stdClass();
+        $raw->{'gcid-x1'} = (object) ['color' => '#fff', 'status' => 'inactive'];
+
+        self::assertSame(ColorBook::fromDivi(['gcid-x1' => ['color' => '#fff', 'status' => 'inactive']])->data(),
+            ColorBook::fromDivi($raw)->data());
+    }
+
+    public function testCustomizerLastUpdatedIsNormalizedSoRevStaysStable(): void
+    {
+        $now = ColorBook::fromDivi(self::stored('2026-10-19T08:00:00.000Z'));
+        $later = ColorBook::fromDivi(self::stored('2026-10-19T09:30:00.000Z'));
+
+        self::assertSame('', $now->entry('gcid-primary-color')['lastUpdated']);
+        self::assertSame('2026-09-27T22:21:39.000Z', $now->entry('gcid-q9swzy97df')['lastUpdated']);
+        self::assertSame($now->rev(), $later->rev());
+        self::assertSame($now->data(), $later->data());
+        self::assertSame([], $later->changedSince($now));
+    }
+
+    public function testRevIgnoresTheOrderOfEntriesAndChangesWithTheContent(): void
+    {
+        $stored = self::stored();
+        $book = ColorBook::fromDivi($stored);
+        $reversed = ColorBook::fromDivi(array_reverse($stored, true));
+
+        self::assertMatchesRegularExpression('/^[0-9a-f]{12}$/', $book->rev());
+        self::assertSame($book->rev(), $reversed->rev());
+        self::assertSame($book->rev(), ColorBook::fromDivi($stored)->rev());
+        self::assertNotSame($book->rev(), $book->with('gcid-q9swzy97df', ['color' => '#000000'])->rev());
+    }
+
+    public function testRealDiviFixtureAndTheSnapshotOfItAreTheSame(): void
+    {
+        $json = (string) file_get_contents(dirname(__DIR__, 4) . '/Fixtures/divi/stage4/global-colors.json');
+        $fixture = json_decode($json, true);
+        $read = ColorBook::fromDivi($fixture['get_global_colors']);
+        $snapshot = ColorBook::fromDivi($read->data());
+
+        self::assertCount(9, $read->data());
+        self::assertSame($read->rev(), $snapshot->rev());
+        self::assertTrue($read->isCustomizer('gcid-link-color'));
+        self::assertFalse($read->isCustomizer('gcid-q9swzy97df'));
+        self::assertSame(['gcid-989stx15zc'], $read->referencing('gcid-q9swzy97df'));
+        self::assertNull($read->cycle());
+    }
+
+    public function testIsCustomizerByIdOrFolder(): void
+    {
+        $book = ColorBook::fromDivi([
+            'gcid-body-color' => ['color' => '#666'],
+            'gcid-legacy' => ['color' => '#666', 'folder' => 'customizer'],
+            'gcid-x1' => ['color' => '#666'],
+        ]);
+
+        self::assertTrue($book->isCustomizer('gcid-body-color'));
+        self::assertTrue($book->isCustomizer('gcid-legacy'));
+        self::assertFalse($book->isCustomizer('gcid-x1'));
+        self::assertTrue($book->isCustomizer('gcid-heading-color'), 'a customizer ID counts even if it is missing');
+        self::assertSame('', $book->entry('gcid-legacy')['lastUpdated']);
+    }
+
+    public function testFindByIdAliasOptionNameAndLabel(): void
+    {
+        $book = ColorBook::fromDivi(self::stored());
+
+        self::assertSame('gcid-q9swzy97df', $book->find('gcid-q9swzy97df'));
+        self::assertSame('gcid-primary-color', $book->find('primary'));
+        self::assertSame('gcid-primary-color', $book->find('PRIMARY'));
+        self::assertSame('gcid-secondary-color', $book->find('secondary_accent_color'));
+        self::assertSame('gcid-q9swzy97df', $book->find('  holz DUNKEL '));
+        self::assertSame('gcid-primary-color', $book->find('Primärfarbe'));
+        self::assertNull($book->find('heading'), 'an alias whose color does not exist finds nothing');
+        self::assertNull($book->find('Fehlt'));
+        self::assertNull($book->find(''));
+    }
+
+    public function testAmbiguousLabelListsTheIds(): void
+    {
+        $this->expectException(InvalidArgumentException::class);
+        $this->expectExceptionMessage(
+            'Name der globalen Farbe "Doppelt" ist mehrdeutig; bitte die ID angeben: gcid-aaaaaaaaaa, gcid-bbbbbbbbbb.'
+        );
+
+        ColorBook::fromDivi(self::stored())->find('Doppelt');
+    }
+
+    public function testWithAddsOrReplacesAndLeavesTheOriginalAlone(): void
+    {
+        $book = ColorBook::fromDivi(self::stored());
+        $added = $book->with('gcid-new0000001', ['color' => '#abc', 'label' => 'Neu']);
+        $replaced = $book->with('gcid-q9swzy97df', ['color' => '#000000', 'label' => 'Holz', 'status' => 'inactive']);
+
+        self::assertCount(6, $book->data());
+        self::assertCount(7, $added->data());
+        self::assertSame('active', $added->entry('gcid-new0000001')['status']);
+        self::assertSame('inactive', $replaced->entry('gcid-q9swzy97df')['status']);
+        self::assertSame('#5b3a29', $book->entry('gcid-q9swzy97df')['color']);
+        self::assertSame('', $book->with('gcid-primary-color', ['color' => '#fff', 'lastUpdated' => 'x'])
+            ->entry('gcid-primary-color')['lastUpdated']);
+    }
+
+    public function testReferencingFindsRelativeColorsIncludingSelfReferences(): void
+    {
+        $book = ColorBook::fromDivi(self::stored())
+            ->with('gcid-self000001', ['color' => self::ref('gcid-self000001')])
+            ->with('gcid-text000001', ['color' => 'Hinweis: gcid-q9swzy97df']);
+
+        self::assertSame(['gcid-989stx15zc'], $book->referencing('gcid-q9swzy97df'));
+        self::assertSame(['gcid-self000001'], $book->referencing('gcid-self000001'));
+        self::assertSame([], $book->referencing('gcid-989stx15zc'));
+    }
+
+    public function testCycleFindsLoopsAndSelfReferences(): void
+    {
+        $loop = ColorBook::fromDivi([
+            'gcid-a' => ['color' => self::ref('gcid-b')],
+            'gcid-b' => ['color' => self::ref('gcid-c', ['hue' => 10])],
+            'gcid-c' => ['color' => self::ref('gcid-b')],
+        ]);
+        $self = ColorBook::fromDivi(['gcid-a' => ['color' => self::ref('gcid-a')]]);
+
+        self::assertSame(['gcid-a', 'gcid-b', 'gcid-c', 'gcid-b'], $loop->cycle());
+        self::assertSame(['gcid-a', 'gcid-a'], $self->cycle());
+        self::assertNull(ColorBook::fromDivi(self::stored())->cycle());
+    }
+
+    public function testCycleStopsAtMoreThanTenReferences(): void
+    {
+        $chain = static function (int $references): ColorBook {
+            $raw = [];
+            for ($i = 0; $i < $references; ++$i) {
+                $raw['gcid-c' . $i] = ['color' => self::ref('gcid-c' . ($i + 1))];
+            }
+            $raw['gcid-c' . $references] = ['color' => '#123456'];
+
+            return ColorBook::fromDivi($raw);
+        };
+
+        self::assertNull($chain(10)->cycle(), 'ten references still resolve in Divi');
+        $tooDeep = $chain(11)->cycle();
+        self::assertCount(12, $tooDeep);
+        self::assertSame(['gcid-c0', 'gcid-c1'], array_slice($tooDeep, 0, 2));
+    }
+
+    public function testReferenceToAnUnknownColorIsNoCycle(): void
+    {
+        self::assertNull(ColorBook::fromDivi(['gcid-a' => ['color' => self::ref('gcid-missing')]])->cycle());
+    }
+
+    public function testChangedSinceListsNewAndChangedEntriesOnly(): void
+    {
+        $before = ColorBook::fromDivi(self::stored());
+        $after = $before
+            ->with('gcid-q9swzy97df', ['color' => '#000000', 'label' => 'Holz dunkel', 'lastUpdated' => 'x'])
+            ->with('gcid-new0000001', ['color' => '#abc', 'label' => 'Neu'])
+            ->with('gcid-aaaaaaaaaa', $before->entry('gcid-aaaaaaaaaa') ?? []);
+
+        self::assertSame(['gcid-q9swzy97df', 'gcid-new0000001'], array_keys($after->changedSince($before)));
+        self::assertSame([], $before->changedSince($before));
+        self::assertSame(['gcid-primary-color'], array_keys(
+            $before->with('gcid-primary-color', ['color' => '#ff0000', 'folder' => 'customizer'])->changedSince($before)
+        ));
+    }
+
+    public function testNewIdHasTheBuilderFormat(): void
+    {
+        self::assertMatchesRegularExpression('/^gcid-[0-9a-z]{10}$/', ColorBook::newId());
+        self::assertNotSame(ColorBook::newId(), ColorBook::newId());
+        self::assertSame(
+            'gcid-abcdefghij',
+            ColorBook::newId(static fn (int $length): string => substr('abcdefghijkl', 0, $length))
+        );
+    }
+
+    public function testNowHasDiviTimestampFormat(): void
+    {
+        self::assertMatchesRegularExpression('/^\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}\.\d{3}Z$/', ColorBook::now());
+    }
+}