diff --git a/include/ptk_color.h b/include/ptk_color.h
index ce9fc62..d5a5c7f 100644
--- a/include/ptk_color.h
+++ b/include/ptk_color.h
@@ -1,6 +1,7 @@
 #ifndef PTK_PTK_COLOR_H_
 #define PTK_PTK_COLOR_H_
 
+// all values in range [0.0, 1.0]
 typedef union {
     struct {
         float r;