Is there an easy way to increase the number of PWM outputs? I want to be able to control 4 x RGB LED's, which would need 12 PWM outputs.
The AVR chip used in your Arduino only but a limited number of PWM outputs available. Some of them can't be used because the associated timer is already being used for other purposes. The only alternatives, then, are to implement PWM in software (not very precise) or add an external chip that implements PWM. The Pak V from Al Williams Computing is one example of such a chip but there may be others as well.
Looks great, but is pretty expensive. Sure it would allow an extra 3 RGB LED's to be connected but it's more expensive than the Arduino itself.
Check into the TLC5940 chip, it's been discussed many times in these forums and there's a library for it.
Or...click my sig.
You can do multiplexing with PWM outputs driving the individual LEDs, but it might be a bit tricky. Ideally, you want the multiplex frequency to be much lower than the PWM frequency.
Thanks, I've order 2 x TLC5940 chips to have a play with.