diff --git a/src/gfx_layout.h b/src/gfx_layout.h
--- a/src/gfx_layout.h
+++ b/src/gfx_layout.h
@@ -47,7 +47,8 @@ struct FontState {
 	 */
 	inline void SetColour(TextColour c)
 	{
-		assert(c >= TC_BLUE && c <= TC_BLACK);
+		assert((c & TC_COLOUR_MASK) >= TC_BLUE && (c & TC_COLOUR_MASK) <= TC_BLACK);
+		assert((c & (TC_COLOUR_MASK | TC_FLAGS_MASK)) == c);
 		if ((this->cur_colour & TC_FORCED) == 0) this->cur_colour = c;
 	}
 
diff --git a/src/gfx_type.h b/src/gfx_type.h
--- a/src/gfx_type.h
+++ b/src/gfx_type.h
@@ -279,6 +279,9 @@ enum TextColour {
 	TC_IS_PALETTE_COLOUR = 0x100, ///< Colour value is already a real palette colour index, not an index of a StringColour.
 	TC_NO_SHADE          = 0x200, ///< Do not add shading to this text colour.
 	TC_FORCED            = 0x400, ///< Ignore colour changes from strings.
+
+	TC_COLOUR_MASK = 0xFF, ///< Mask to test if TextColour (without flags) is within limits.
+	TC_FLAGS_MASK = 0x700, ///< Mask to test if TextColour (with flags) is within limits.
 };
 DECLARE_ENUM_AS_BIT_SET(TextColour)