-#define COLOUR_X11_(r, g, b) "#"#r#g#b
-#define COLOUR_X11(c) COLOUR_X11_ c
+#ifndef COLOUR_SYSTEM
+# define COLOUR_SYSTEM X11
+#endif
+
+#define COLOUR_SYS_X11(r, g, b) "#"#r#g#b
+
+#define COLOUR_SYS__(mode, c) COLOUR_SYS_##mode c
+#define COLOUR_SYS_(mode, c) COLOUR_SYS__(mode, c)
+#define COLOUR_SYS(c) COLOUR_SYS_(COLOUR_SYSTEM, c)