diff --git a/src/js/util.ts b/src/js/util.ts
index 008e584..93c1fa3 100644
--- a/src/js/util.ts
+++ b/src/js/util.ts
@@ -31,6 +31,8 @@ const DEG_HALF = 180;
 
 /* regexp */
 const REG_COLOR = new RegExp(`^(?:${SYN_COLOR_TYPE})$`);
+const REG_FN_COLOR =
+  /^(?:(?:ok)?l(?:ab|ch)|color(?:-mix)?|hsla?|hwb|rgba?|var)\(/;
 const REG_MIX = new RegExp(SYN_MIX);
 
 /**
@@ -175,7 +177,7 @@ export const isColor = (value: unknown, opt: Options = {}): boolean => {
         }
       } else if (REG_COLOR.test(value) || REG_MIX.test(value)) {
         return true;
-      } else {
+      } else if (REG_FN_COLOR.test(value)) {
         opt.nullable = true;
         if (!opt.format) {
           opt.format = VAL_SPEC;
diff --git a/test/css-gradient.test.ts b/test/css-gradient.test.ts
index 3933eb3..cc3fa49 100644
--- a/test/css-gradient.test.ts
+++ b/test/css-gradient.test.ts
@@ -411,4 +411,21 @@ describe('is CSS gradient', () => {
     const res = func('linear-gradient(red, blue)');
     assert.strictEqual(res, true, 'result');
   });
+
+  it('should get true', () => {
+    const res = func('radial-gradient(transparent, var(--custom-color))');
+    assert.strictEqual(res, true, 'result');
+  });
+
+  it('should get true', () => {
+    const res = func('radial-gradient(transparent, var(--custom-color))');
+    assert.strictEqual(res, true, 'result');
+  });
+
+  it('should get true', () => {
+    const res = func(
+      'radial-gradient(transparent, /* comment */ var(--custom-color))'
+    );
+    assert.strictEqual(res, true, 'result');
+  });
 });
diff --git a/test/util.test.ts b/test/util.test.ts
index 283c7f5..364e464 100644
--- a/test/util.test.ts
+++ b/test/util.test.ts
@@ -190,6 +190,16 @@ describe('is color', () => {
     );
     assert.strictEqual(res, true, 'result');
   });
+
+  it('should get false', () => {
+    const res = func('url(var(--foo))');
+    assert.strictEqual(res, false, 'result');
+  });
+
+  it('should get false', () => {
+    const res = func('radial-gradient(transparent, var(--custom-color))');
+    assert.strictEqual(res, false, 'result');
+  });
 });
 
 describe('value to JSON string', () => {