Some time ago while talking with someone else about this project, the idea came to use RGB LEDs. And there are small suitable RGB LEDs available: 0404 RGB LEDs. So this is what I came up with: 21*8 RGB LEDs ( =168 RGB LEDs or 504 single LED chips):

Problem here is how to control so many LEDs? I am not sure yet but I am planing to multiplex the lines. And driving the columns via some (21*3=63 Bit!) shift registers. I am also thinking of using a Microcontroller which is faster than a ATmega to get a acceptable refresh rate on the Matrix.

The PCBs measurements did not changed much. The goal is still to fit this and a control PCB of the same size into 5x5cm:

Because this project is still in early stage it is possible that I will return to a single color version ( or never built one ).