diff --git a/src/builtin/color.rs b/src/builtin/color.rs
index d1ab180..e464aec 100644
--- a/src/builtin/color.rs
+++ b/src/builtin/color.rs
@@ -151,8 +151,13 @@ pub(crate) fn register(f: &mut BTreeMap<String, Builtin>) {
         }
     });
     decl!(f "invert", |args, _| {
+        let weight = match arg!(args, 1, "weight"=Value::Dimension(Number::from(100), Unit::Percent)) {
+            Value::Dimension(n, Unit::None)
+            | Value::Dimension(n, Unit::Percent) => n / Number::from(100),
+            _ => todo!("non-number weight in given to builtin function `invert()`")
+        };
         match arg!(args, 0, "color") {
-            Value::Color(c) => Some(Value::Color(c.invert())),
+            Value::Color(c) => Some(Value::Color(c.invert(weight))),
             _ => todo!("non-color given to builtin function `alpha()`")
         }
     });
diff --git a/src/builtin/macros.rs b/src/builtin/macros.rs
index 861fdf0..51c8bc5 100644
--- a/src/builtin/macros.rs
+++ b/src/builtin/macros.rs
@@ -8,11 +8,11 @@ macro_rules! arg {
             },
         };
     };
-    ($args:ident, $idx:literal, $name:literal=$default:literal) => {
+    ($args:ident, $idx:literal, $name:literal=$default:expr) => {
         match $args.get(stringify!($idx)) {
-            Some(v) => v,
+            Some(v) => v.clone(),
             None => match $args.get($name) {
-                Some(v) => v,
+                Some(v) => v.clone(),
                 None => $default,
             },
         };
diff --git a/src/color/mod.rs b/src/color/mod.rs
index 52cdfc4..c7c65fc 100644
--- a/src/color/mod.rs
+++ b/src/color/mod.rs
@@ -229,10 +229,17 @@ impl Color {
         }
     }
 
-    pub fn invert(&self) -> Self {
-        let red = std::u8::MAX - self.red;
-        let green = std::u8::MAX - self.green;
-        let blue = std::u8::MAX - self.blue;
+    pub fn invert(&self, weight: Number) -> Self {
+        let weight = if weight > Number::from(1) {
+            Number::from(1)
+        } else if weight < Number::from(0) {
+            Number::from(0)
+        } else {
+            weight
+        };
+        let red = (Number::from(std::u8::MAX) - (Number::from(self.red) * weight.clone())).round().to_integer().to_u8().unwrap();
+        let green = (Number::from(std::u8::MAX) - (Number::from(self.green) * weight.clone())).round().to_integer().to_u8().unwrap();
+        let blue = (Number::from(std::u8::MAX) - (Number::from(self.blue) * weight)).round().to_integer().to_u8().unwrap();
         let repr = repr(red, green, blue, &self.alpha);
         Color {
             red,
diff --git a/tests/color.rs b/tests/color.rs
index b0e92e1..1fe008a 100644
--- a/tests/color.rs
+++ b/tests/color.rs
@@ -176,3 +176,13 @@ test!(
     "a {\n  color: invert(white);\n}\n",
     "a {\n  color: black;\n}\n"
 );
+test!(
+    invert_weight_percent,
+    "a {\n  color: invert(white, 20%);\n}\n",
+    "a {\n  color: #cccccc;\n}\n"
+);
+test!(
+    invert_weight_no_unit,
+    "a {\n  color: invert(white, 20);\n}\n",
+    "a {\n  color: #cccccc;\n}\n"
+);