Try creating colors by time multiplexing, combine that with intensity regulation (timing again), combine that with scrolling (timing again), combine that with 'effects' (timing again), and then throw in multiplexing (more timing). and, of course in all this timing DO remember that if the whole thing is not done within 8ms someone will tell you that your sign is 'blinking'. (someone: I did a totally unscientific investigation and all but one saw no 'blibking' at 10ms, but one still saw it). Now, of course if your sign had 7 LEDs this is fairly simple, but try it with a 5000 LED sign.

Anbother factor is that you need much higher current ratings for your LEDs and your drivers thus these components become more exprensive and the much lauded cost savings by multiplexing almost disappear.

Were I to design a single color sign with no (or minimal) 'effects' I might have a look, but otherwise ...

Erik