one thing i found which helps was to go to rits website and use their color guide/recipe thing (you choose a color, it will tell you how to mix their colors to get it).  sometimes the mixes look WAY off, but amazingly they dye the way they were supposed to color.  but not always.

also take note acetone is a toxic chemical and could be detrimental to your health.  work in a well ventilated area with respiratory protection, and be weary that when you add acetone to already near-boiling water, the acetone lowers the boiling temp and the water will start to boil, you might get splash-back.

so, add acetone to water, not water to the acetone.  i've read supposedly that the material you pour into is the one which tends to splash back.

i love rit dying, you've just got to get the hang of it first.  once you do you can get some remarkable results with no noticeable weakening in the plastic.

the main drawback to it (for me) is that it only permeates the top layer of plastic, so you could potentially scratch below the colored layer exposing the original color.  then again, paint does the same thing when scratched.