diff --git a/src/Integrations/Divi/Design/ColorDiff.php b/src/Integrations/Divi/Design/ColorDiff.php
new file mode 100644
index 0000000..b60877e
--- /dev/null
+++ b/src/Integrations/Divi/Design/ColorDiff.php
@@ -0,0 +1,86 @@
+<?php
+
+declare(strict_types=1);
+
+namespace LIT\MCP\Integrations\Divi\Design;
+
+defined('ABSPATH') || exit;
+
+/**
+ * What a color change does (spec §4 step 4, "diff"): per color added, changed (with the changed fields), deactivated
+ * or activated (status change; "fields" lists further changed fields), removed (only a restore can drop a color).
+ * lastUpdated and usedInPosts are no change of their own. Sorted by ID. Pure PHP.
+ */
+final class ColorDiff
+{
+    private const COMPARED = ['color', 'folder', 'label', 'status'];
+
+    /**
+     * @return list<array{id: string, name: string, action: string, fields?: list<string>}>
+     *     action: added|changed|deactivated|activated|removed
+     */
+    public static function between(ColorBook $before, ColorBook $after): array
+    {
+        $ids = array_unique(array_merge(array_keys($before->data()), array_keys($after->data())));
+        sort($ids);
+        $rows = [];
+        foreach ($ids as $id) {
+            $id = (string) $id;
+            $was = $before->entry($id);
+            $is = $after->entry($id);
+            if ($was === null || $is === null) {
+                $book = $is === null ? $before : $after;
+                $rows[] = ['id' => $id, 'name' => $book->label($id), 'action' => $is === null ? 'removed' : 'added'];
+                continue;
+            }
+            $action = self::action($was['status'], $is['status']);
+            // A status switch is the action itself; "fields" then names only the other changes.
+            $skip = $action === 'changed' ? [] : ['status'];
+            $fields = array_values(array_filter(
+                self::COMPARED,
+                static fn (string $key): bool => $was[$key] !== $is[$key] && ! in_array($key, $skip, true)
+            ));
+            $row = ['id' => $id, 'name' => $after->label($id), 'action' => $action];
+            if ($fields !== []) {
+                $row['fields'] = $fields;
+            }
+            if ($fields !== [] || $action !== 'changed') {
+                $rows[] = $row;
+            }
+        }
+
+        return $rows;
+    }
+
+    /**
+     * One German sentence for the dry run and the change log (spec §4 step 6).
+     *
+     * @param list<array<string, mixed>> $diff
+     */
+    public static function summary(array $diff): string
+    {
+        $count = array_count_values(array_map(static fn (array $row): string => (string) $row['action'], $diff));
+        $words = [
+            'added' => 'angelegt', 'changed' => 'geändert', 'deactivated' => 'deaktiviert',
+            'activated' => 'aktiviert', 'removed' => 'gelöscht',
+        ];
+        $parts = [];
+        foreach ($words as $action => $word) {
+            if (isset($count[$action])) {
+                $parts[] = $count[$action] . ' ' . $word;
+            }
+        }
+
+        return 'Farben: ' . ($parts === [] ? 'keine Änderung' : implode(', ', $parts)) . '.';
+    }
+
+    private static function action(string $was, string $is): string
+    {
+        return match (true) {
+            $was === $is => 'changed',
+            $is === 'inactive' => 'deactivated',
+            $is === 'active' => 'activated',
+            default => 'changed',
+        };
+    }
+}
diff --git a/tests/Unit/Integrations/Divi/Design/ColorDiffTest.php b/tests/Unit/Integrations/Divi/Design/ColorDiffTest.php
new file mode 100644
index 0000000..98e4759
--- /dev/null
+++ b/tests/Unit/Integrations/Divi/Design/ColorDiffTest.php
@@ -0,0 +1,79 @@
+<?php
+
+declare(strict_types=1);
+
+namespace LIT\MCP\Tests\Unit\Integrations\Divi\Design;
+
+use LIT\MCP\Integrations\Divi\Design\ColorBook;
+use LIT\MCP\Integrations\Divi\Design\ColorDiff;
+use PHPUnit\Framework\TestCase;
+
+final class ColorDiffTest extends TestCase
+{
+    public function testNoChangeIsAnEmptyDiffEvenWhenOnlyDerivedFieldsDiffer(): void
+    {
+        $book = ColorBook::fromDivi(ColorBookTest::stored());
+        $touched = $book->with('gcid-q9swzy97df', [
+            'color' => '#5b3a29', 'label' => 'Holz dunkel', 'lastUpdated' => '2030-01-01T00:00:00.000Z',
+            'usedInPosts' => [7],
+        ]);
+
+        self::assertSame([], ColorDiff::between($book, ColorBook::fromDivi(ColorBookTest::stored())));
+        self::assertSame([], ColorDiff::between($book, $touched));
+    }
+
+    public function testAddedChangedDeactivatedActivatedAndRemoved(): void
+    {
+        $before = ColorBook::fromDivi(ColorBookTest::stored());
+        $after = $before
+            ->with('gcid-new0000001', ['color' => '#abc', 'label' => 'Neu'])
+            ->with('gcid-q9swzy97df', ['color' => '#000000', 'label' => 'Holz dunkler'])
+            ->with('gcid-aaaaaaaaaa', ['color' => '#111111', 'label' => 'Doppelt', 'status' => 'inactive'])
+            ->with('gcid-bbbbbbbbbb', ['color' => '#222222', 'label' => 'doppelt', 'status' => 'active'])
+            ->with('gcid-989stx15zc', ['color' => '#333333', 'label' => 'Holz hell', 'status' => 'temporary']);
+
+        self::assertSame([
+            ['id' => 'gcid-989stx15zc', 'name' => 'Holz hell', 'action' => 'changed', 'fields' => ['color', 'status']],
+            ['id' => 'gcid-aaaaaaaaaa', 'name' => 'Doppelt', 'action' => 'deactivated'],
+            ['id' => 'gcid-bbbbbbbbbb', 'name' => 'doppelt', 'action' => 'activated'],
+            ['id' => 'gcid-new0000001', 'name' => 'Neu', 'action' => 'added'],
+            ['id' => 'gcid-q9swzy97df', 'name' => 'Holz dunkler', 'action' => 'changed',
+                'fields' => ['color', 'label']],
+        ], ColorDiff::between($before, $after));
+
+        $restored = ColorBook::fromDivi(array_diff_key(ColorBookTest::stored(), ['gcid-aaaaaaaaaa' => 1]));
+        self::assertSame(
+            [['id' => 'gcid-aaaaaaaaaa', 'name' => 'Doppelt', 'action' => 'removed']],
+            ColorDiff::between($before, $restored)
+        );
+    }
+
+    public function testStatusSwitchWithFurtherChangesNamesTheOtherFields(): void
+    {
+        $before = ColorBook::fromDivi(ColorBookTest::stored());
+        $after = $before->with('gcid-aaaaaaaaaa', [
+            'color' => '#999999', 'label' => 'Doppelt', 'folder' => 'Marke', 'status' => 'inactive',
+        ]);
+
+        self::assertSame(
+            [['id' => 'gcid-aaaaaaaaaa', 'name' => 'Doppelt', 'action' => 'deactivated',
+                'fields' => ['color', 'folder']]],
+            ColorDiff::between($before, $after)
+        );
+    }
+
+    public function testSummaryCountsTheActions(): void
+    {
+        self::assertSame(
+            'Farben: 1 angelegt, 2 geändert, 1 deaktiviert.',
+            ColorDiff::summary([
+                ['action' => 'added'], ['action' => 'changed'], ['action' => 'changed'], ['action' => 'deactivated'],
+            ])
+        );
+        self::assertSame(
+            'Farben: 1 geändert, 1 aktiviert, 1 gelöscht.',
+            ColorDiff::summary([['action' => 'removed'], ['action' => 'activated'], ['action' => 'changed']])
+        );
+        self::assertSame('Farben: keine Änderung.', ColorDiff::summary([]));
+    }
+}