diff --git a/src/builtin/color/hsl.rs b/src/builtin/color/hsl.rs
index c7accf3..1cee31f 100644
--- a/src/builtin/color/hsl.rs
+++ b/src/builtin/color/hsl.rs
@@ -73,7 +73,7 @@ pub(crate) fn register(f: &mut BTreeMap<String, Builtin>) {
         };
         let degrees = match arg!(args, 1, "degrees").eval() {
             Value::Dimension(n, _) => n,
-            _ => todo!("expected either unitless or % number for degrees"),
+            v => return Err(format!("$degrees: {} is not a number.", v).into()),
         };
         Ok(Value::Color(color.adjust_hue(degrees)))
     });
@@ -137,9 +137,8 @@ pub(crate) fn register(f: &mut BTreeMap<String, Builtin>) {
     });
     decl!(f "invert", |args, _| {
         let weight = match arg!(args, 1, "weight"=Value::Dimension(Number::from(100), Unit::Percent)) {
-            Value::Dimension(n, Unit::None)
-            | Value::Dimension(n, Unit::Percent) => n / Number::from(100),
-            _ => todo!("non-number weight given to builtin function `invert()`")
+            Value::Dimension(n, _) => n / Number::from(100),
+            v => return Err(format!("$weight: {} is not a number.", v).into()),
         };
         match arg!(args, 0, "color") {
             Value::Color(c) => Ok(Value::Color(c.invert(weight))),
diff --git a/src/builtin/color/opacity.rs b/src/builtin/color/opacity.rs
index 0bba625..469c7c1 100644
--- a/src/builtin/color/opacity.rs
+++ b/src/builtin/color/opacity.rs
@@ -3,7 +3,7 @@ use std::collections::BTreeMap;
 use super::Builtin;
 use crate::common::QuoteKind;
 use crate::units::Unit;
-use crate::value::{Number, Value};
+use crate::value::Value;
 
 pub(crate) fn register(f: &mut BTreeMap<String, Builtin>) {
     decl!(f "alpha", |args, _| {
@@ -25,9 +25,8 @@ pub(crate) fn register(f: &mut BTreeMap<String, Builtin>) {
             v => return Err(format!("$color: {} is not a color.", v).into()),
         };
         let amount = match arg!(args, 1, "amount").eval() {
-            Value::Dimension(n, Unit::None) => n,
-            Value::Dimension(n, Unit::Percent) => n / Number::from(100),
-            _ => todo!("expected either unitless or % number for amount"),
+            Value::Dimension(n, _) => n,
+            v => return Err(format!("$amount: {} is not a number.", v).into()),
         };
         Ok(Value::Color(color.fade_in(amount)))
     });
@@ -37,9 +36,8 @@ pub(crate) fn register(f: &mut BTreeMap<String, Builtin>) {
             v => return Err(format!("$color: {} is not a color.", v).into()),
         };
         let amount = match arg!(args, 1, "amount").eval() {
-            Value::Dimension(n, Unit::None) => n,
-            Value::Dimension(n, Unit::Percent) => n / Number::from(100),
-            _ => todo!("expected either unitless or % number for amount"),
+            Value::Dimension(n, _) => n,
+            v => return Err(format!("$amount: {} is not a number.", v).into()),
         };
         Ok(Value::Color(color.fade_in(amount)))
     });
@@ -49,9 +47,8 @@ pub(crate) fn register(f: &mut BTreeMap<String, Builtin>) {
             v => return Err(format!("$color: {} is not a color.", v).into()),
         };
         let amount = match arg!(args, 1, "amount").eval() {
-            Value::Dimension(n, Unit::None) => n,
-            Value::Dimension(n, Unit::Percent) => n / Number::from(100),
-            _ => todo!("expected either unitless or % number for amount"),
+            Value::Dimension(n, _) => n,
+            v => return Err(format!("$amount: {} is not a number.", v).into()),
         };
         Ok(Value::Color(color.fade_out(amount)))
     });
@@ -61,9 +58,8 @@ pub(crate) fn register(f: &mut BTreeMap<String, Builtin>) {
             v => return Err(format!("$color: {} is not a color.", v).into()),
         };
         let amount = match arg!(args, 1, "amount").eval() {
-            Value::Dimension(n, Unit::None) => n,
-            Value::Dimension(n, Unit::Percent) => n / Number::from(100),
-            _ => todo!("expected either unitless or % number for amount"),
+            Value::Dimension(n, _) => n,
+            v => return Err(format!("$amount: {} is not a number.", v).into()),
         };
         Ok(Value::Color(color.fade_out(amount)))
     });
diff --git a/src/builtin/color/rgb.rs b/src/builtin/color/rgb.rs
index 8d5d306..c554c8b 100644
--- a/src/builtin/color/rgb.rs
+++ b/src/builtin/color/rgb.rs
@@ -117,7 +117,7 @@ pub(crate) fn register(f: &mut BTreeMap<String, Builtin>) {
         } else if args.len() == 2 {
             let color = match arg!(args, 0, "color").eval() {
                 Value::Color(c) => c,
-                _ => todo!("expected color")
+                v => return Err(format!("$color: {} is not a color.", v).into()),
             };
             let alpha = match arg!(args, 1, "alpha").eval() {
                 Value::Dimension(n, Unit::None) => n,
@@ -183,10 +183,9 @@ pub(crate) fn register(f: &mut BTreeMap<String, Builtin>) {
             v => return Err(format!("$color: {} is not a color.", v).into()),
         };
 
-        let weight = match arg!(args, 2, "weight"=Value::Dimension(Number::ratio(1, 2), Unit::None)) {
-            Value::Dimension(n, Unit::None) => n,
-            Value::Dimension(n, Unit::Percent) => n / Number::from(100),
-            _ => todo!("expected either unitless or % number for $weight")
+        let weight = match arg!(args, 2, "weight"=Value::Dimension(Number::from(50), Unit::None)) {
+            Value::Dimension(n, _) => n / Number::from(100),
+            v => return Err(format!("$weight: {} is not a number.", v).into()),
         };
         Ok(Value::Color(color1.mix(color2, weight)))
     });