I made 30 groups of particles, for each group setup own PShape with material (30 mat, 30 object, 30 TPgroup). And change group depending there speed. If you dont understood you should watch video.
0:22 - Setup simple emitter
3:05 - Control Scale depending on velocity.
5:30 - Convert Velocity(Vector) to Speed(real)
11:00 - Apply colors from image on mat.
16:40 - Setup xpresso for color changer.
22:10 - Making of "Liquid test 1".
I hope this was helpful!
UPD: The easier way vimeo.com/37117453#comment_6684731
UPD 2: Easiest way to colorize particles based on velocity plugins4cinema4d.com/pshade.html
Also check out other tutorials on my web site philipppavlov.com/tutorials