style: use one spelling for colour across the CLI

Assisted-by: GLM 5.3 Flash

cmd/gasm/main.go

@@ -102,7 +102,7 @@ func cmdVersion() int {
 	return 0
 }
 
-// ANSI color helpers for terminal output.
+// ANSI colour helpers for terminal output.
 const (
 	colorReset  = "\033[0m"
 	colorBold   = "\033[1m"
@@ -111,7 +111,7 @@ const (
 	colorGray   = "\033[90m"
 )
 
-// isTTY reports whether the writer is a terminal (for color output).
+// isTTY reports whether the writer is a terminal (for colour output).
 func isTTY(w io.Writer) bool {
 	if f, ok := w.(*os.File); ok {
 		stat, _ := f.Stat()