I am still not sure what project to use my rgb 7-segment display for. There were some very interesting ideas posted to the Hackaday forum. I like matthiasrs idea most: A 2-digit clock, using the red LEDs for hours, the green ones for minutes and the blue ones for seconds.
In order to get a better idea, rather then implementing it in hardware I wrote a little simulation as Java applet. Have a look and see how hard or easy it is to identify the time