no your right, but how much your able to mix them is a function of what your using to drive them, and not necessarily the led
If you were just to turn on red and blue, you would get purple, but maxing out at about 6 unique colors
if your able to half turn on red and fully turn on blue now you have a new shade of purple, but increased complexity in order to drive the red led (or whatever one) at 50%
Its a choice for you to make how many colors you want to have, and how much wiring your willing to put up with