diff --git a/src/gfx_type.h b/src/gfx_type.h
index 622676b5a0..3e08001ad9 100644
--- a/src/gfx_type.h
+++ b/src/gfx_type.h
@@ -219,7 +219,7 @@ struct SubSprite {
 	int left, top, right, bottom;
 };
 
-enum Colours {
+enum Colours : uint8 {
 	COLOUR_BEGIN,
 	COLOUR_DARK_BLUE = COLOUR_BEGIN,
 	COLOUR_PALE_GREEN,