ArdourCanvas::Color c = ArdourCanvas::contrasting_text_color (ArdourCanvas::rgba_to_color (r, g, b, a));
ArdourCanvas::color_to_rgba (c, r, g, b, a);
ArdourCanvas::Color c = ArdourCanvas::contrasting_text_color (ArdourCanvas::rgba_to_color (r, g, b, a));
ArdourCanvas::color_to_rgba (c, r, g, b, a);