diff --git a/api/src/Entity/Category.php b/api/src/Entity/Category.php
index 23937d2e17..c5ac7b2a4e 100644
--- a/api/src/Entity/Category.php
+++ b/api/src/Entity/Category.php
@@ -143,6 +143,8 @@ class Category extends BaseEntity implements BelongsToCampInterface, CopyFromPro
     /**
      * The color of the activities in this category, as a hex color string.
      */
+    #[InputFilter\Trim]
+    #[Assert\NotBlank]
     #[Assert\Regex(pattern: '/^#[0-9a-zA-Z]{6}$/')]
     #[ApiProperty(example: '#4DBB52')]
     #[Groups(['read', 'write'])]
diff --git a/api/tests/Api/Categories/CreateCategoryTest.php b/api/tests/Api/Categories/CreateCategoryTest.php
index 52fb1ebe81..5433282fb1 100644
--- a/api/tests/Api/Categories/CreateCategoryTest.php
+++ b/api/tests/Api/Categories/CreateCategoryTest.php
@@ -341,6 +341,49 @@ public function testCreateCategoryCleansForbiddenCharactersFromName() {
         ));
     }
 
+    public function testCreateCategoryValidatesNullColor() {
+        static::createClientWithCredentials()->request(
+            'POST',
+            '/categories',
+            [
+                'json' => $this->getExampleWritePayload(
+                    [
+                        'color' => null,
+                    ]
+                ),
+            ]
+        );
+
+        $this->assertResponseStatusCodeSame(400);
+        $this->assertJsonContains([
+            'detail' => 'The type of the "color" attribute must be "string", "NULL" given.',
+        ]);
+    }
+
+    public function testCreateCategoryValidatesEmptyColor() {
+        static::createClientWithCredentials()->request(
+            'POST',
+            '/categories',
+            [
+                'json' => $this->getExampleWritePayload(
+                    [
+                        'color' => '',
+                    ]
+                ),
+            ]
+        );
+
+        $this->assertResponseStatusCodeSame(422);
+        $this->assertJsonContains([
+            'violations' => [
+                [
+                    'propertyPath' => 'color',
+                    'message' => 'This value should not be blank.',
+                ],
+            ],
+        ]);
+    }
+
     public function testCreateCategoryValidatesMissingColor() {
         static::createClientWithCredentials()->request('POST', '/categories', ['json' => $this->getExampleWritePayload([], ['color'])]);
 
@@ -349,7 +392,7 @@ public function testCreateCategoryValidatesMissingColor() {
             'violations' => [
                 [
                     'propertyPath' => 'color',
-                    'message' => 'This value should not be null.',
+                    'message' => 'This value should not be blank.',
                 ],
             ],
         ]);
@@ -371,6 +414,25 @@ public function testCreateCategoryValidatesInvalidColor() {
         ]);
     }
 
+    public function testCreateCategoryTrimsColor() {
+        static::createClientWithCredentials()->request(
+            'POST',
+            '/categories',
+            [
+                'json' => $this->getExampleWritePayload(
+                    [
+                        'color' => " \t #4DBB52 \t ",
+                    ]
+                ),
+            ]
+        );
+
+        $this->assertResponseStatusCodeSame(201);
+        $this->assertJsonContains($this->getExampleReadPayload([
+            'color' => '#4DBB52',
+        ]));
+    }
+
     public function testCreateCategoryUsesDefaultForMissingNumberingStyle() {
         static::createClientWithCredentials()->request('POST', '/categories', ['json' => $this->getExampleWritePayload([], ['numberingStyle'])]);
 
diff --git a/api/tests/Api/Categories/UpdateCategoryTest.php b/api/tests/Api/Categories/UpdateCategoryTest.php
index 3b5a858f0c..b9b51146a6 100644
--- a/api/tests/Api/Categories/UpdateCategoryTest.php
+++ b/api/tests/Api/Categories/UpdateCategoryTest.php
@@ -416,6 +416,49 @@ public function testPatchCategoryCleansForbiddenCharactersFromName() {
         );
     }
 
+    public function testPatchCategoryValidatesNullColor() {
+        $category = static::$fixtures['category1'];
+        static::createClientWithCredentials()->request(
+            'PATCH',
+            '/categories/'.$category->getId(),
+            [
+                'json' => [
+                    'color' => null,
+                ],
+                'headers' => ['Content-Type' => 'application/merge-patch+json'],
+            ]
+        );
+
+        $this->assertResponseStatusCodeSame(400);
+        $this->assertJsonContains([
+            'detail' => 'The type of the "color" attribute must be "string", "NULL" given.',
+        ]);
+    }
+
+    public function testPatchCategoryValidatesEmptyColor() {
+        $category = static::$fixtures['category1'];
+        static::createClientWithCredentials()->request(
+            'PATCH',
+            '/categories/'.$category->getId(),
+            [
+                'json' => [
+                    'color' => '',
+                ],
+                'headers' => ['Content-Type' => 'application/merge-patch+json'],
+            ]
+        );
+
+        $this->assertResponseStatusCodeSame(422);
+        $this->assertJsonContains([
+            'violations' => [
+                [
+                    'propertyPath' => 'color',
+                    'message' => 'This value should not be blank.',
+                ],
+            ],
+        ]);
+    }
+
     public function testPatchCategoryValidatesInvalidColor() {
         $category = static::$fixtures['category1'];
         static::createClientWithCredentials()->request('PATCH', '/categories/'.$category->getId(), ['json' => [
@@ -433,6 +476,25 @@ public function testPatchCategoryValidatesInvalidColor() {
         ]);
     }
 
+    public function testPatchCategoryTrimsColor() {
+        $category = static::$fixtures['category1'];
+        static::createClientWithCredentials()->request(
+            'PATCH',
+            '/categories/'.$category->getId(),
+            [
+                'json' => [
+                    'color' => " \t #4DBB52 \t ",
+                ],
+                'headers' => ['Content-Type' => 'application/merge-patch+json'],
+            ]
+        );
+
+        $this->assertResponseStatusCodeSame(200);
+        $this->assertJsonContains([
+            'color' => '#4DBB52',
+        ]);
+    }
+
     public function testPatchCategoryValidatesInvalidNumberingStyle() {
         $category = static::$fixtures['category1'];
         static::createClientWithCredentials()->request('PATCH', '/categories/'.$category->getId(), ['json' => [