diff --git a/src/builtin/color.rs b/src/builtin/color.rs
index 8d317cb..b932d46 100644
--- a/src/builtin/color.rs
+++ b/src/builtin/color.rs
@@ -3,9 +3,41 @@ use std::collections::BTreeMap;
 use super::Builtin;
 use crate::value::Value;
 
+macro_rules! arg {
+    ($args:ident, $idx:literal, $name:literal) => {
+        match $args.get(stringify!($idx)) {
+            Some(v) => v,
+            None => match $args.get($name) {
+                Some(v) => v,
+                None => panic!("missing variable"),
+            },
+        };
+    };
+    ($args:ident, $idx:literal, $name:literal, $default:literal) => {
+        match $args.get(stringify!($idx)) {
+            Some(v) => v,
+            None => match $args.get($name) {
+                Some(v) => v,
+                None => $default,
+            },
+        };
+    };
+}
+
 pub(crate) fn register(f: &mut BTreeMap<String, Builtin>) {
-    f.insert("rgb".to_owned(), Box::new(|args| {
-        let red = args.get("red");
-        todo!()
-    }));
-}
\ No newline at end of file
+    f.insert(
+        "rgb".to_owned(),
+        Box::new(|args| {
+            let channels = args.get("channels").unwrap_or(&Value::Null);
+            if channels.is_null() {
+                let _red: &Value = arg!(args, 0, "red");
+                let _green: &Value = arg!(args, 1, "green");
+                let _blue: &Value = arg!(args, 2, "blue");
+            // Value::Color()
+            } else {
+                todo!("channels variable in `rgb`")
+            };
+            todo!()
+        }),
+    );
+}
diff --git a/src/builtin/mod.rs b/src/builtin/mod.rs
index 25f2eee..3f4eb52 100644
--- a/src/builtin/mod.rs
+++ b/src/builtin/mod.rs
@@ -2,13 +2,11 @@ use lazy_static::lazy_static;
 use std::collections::BTreeMap;
 
 use crate::args::CallArgs;
-use crate::common::Scope;
-use crate::function::Function;
 use crate::value::Value;
 
 mod color;
 
-pub(crate) type Builtin = Box<dyn Fn(&CallArgs) -> Value + Send + Sync>;
+pub(crate) type Builtin = Box<dyn Fn(&CallArgs) -> Option<Value> + Send + Sync>;
 
 lazy_static! {
     pub(crate) static ref GLOBAL_FUNCTIONS: BTreeMap<String, Builtin> = {
@@ -17,4 +15,3 @@ lazy_static! {
         m
     };
 }
-