If i'm not wrong, chips like 7219 can only output a 1 or 0 state to a pin, so if you use it with RGB leds, you could only have a limited amount of color (27?), not the millions you could have with a PWM output on each pin...
I still haven't found a mux/demux that could send PWM on each output... Something like a serial in and 8 PWM outputs.... that would just be awesome.
But i may be wrong
melka