diff --git a/src/hb-ot-color.h b/src/hb-ot-color.h
index c23ce4de4..d11e07e23 100644
--- a/src/hb-ot-color.h
+++ b/src/hb-ot-color.h
@@ -102,6 +102,10 @@ hb_ot_color_has_layers (hb_face_t *face);
  *
  * Pairs of glyph and color index.
  *
+ * A color index of 0xFFFF does not refer to a palette
+ * color, but indicates that the foreground color should
+ * be used.
+ *
  * Since: 2.1.0
  **/
 typedef struct hb_ot_color_layer_t {