Properly implement weighted inversion

src/builtin/color/hsl.rs

@@ -150,7 +150,7 @@ pub(crate) fn register(f: &mut BTreeMap<String, Builtin>) {
     decl!(f "invert", |args, _| {
         max_args!(args, 2);
         let weight = match arg!(args, 1, "weight"=Value::Dimension(Number::from(100), Unit::Percent)) {
-            Value::Dimension(n, _) => n / Number::from(100),
+            Value::Dimension(n, u) => bound!("amount", n, u, 0, 100) / Number::from(100),
             v => return Err(format!("$weight: {} is not a number.", v).into()),
         };
         match arg!(args, 0, "color") {

src/color/mod.rs

@@ -424,18 +424,15 @@ impl Color {
     }
 
     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(u8::max_value()) - self.red() * weight.clone();
-        let green = Number::from(u8::max_value()) - self.green() * weight.clone();
-        let blue = Number::from(u8::max_value()) - self.blue() * weight;
+        if weight == Number::from(0) {
+            return self.clone();
+        }
+        let red = Number::from(u8::max_value()) - self.red();
+        let green = Number::from(u8::max_value()) - self.green();
+        let blue = Number::from(u8::max_value()) - self.blue();
         let repr = repr(&red, &green, &blue, &self.alpha());
-        Color::new_rgba(red, green, blue, self.alpha(), repr)
+        let inverse = Color::new_rgba(red, green, blue, self.alpha(), repr);
+        inverse.mix(self.clone(), weight)
     }
 
     pub fn complement(&self) -> Self {

tests/color.rs

@@ -263,6 +263,11 @@ test!(
     "a {\n  color: invert(white, 20%);\n}\n",
     "a {\n  color: #cccccc;\n}\n"
 );
+test!(
+    invert_weight_percent_turquoise,
+    "a {\n  color: invert(turquoise, 23%);\n}\n",
+    "a {\n  color: #5db4ab;\n}\n"
+);
 test!(
     invert_weight_no_unit,
     "a {\n  color: invert(white, 20);\n}\n",