RedGiant People have lots of Particular presets to play with. You'll probably never get a true photorealistic result in Particular, but here are some options:
The best way is to film real paint yourself!
Alternately, Footage Firm is selling some paint splash footage very cheap at the moment - About 20 HD clips for under 9 bucks, delivered.
Agaoin with the Particular. Ywan.
Waveworld and Colorama. This combination is calble of simulating the surface and the interaction with some edges if you carefully build the alpha source for Waveworld. You can augment the illusion with Foam.
Fionding tutorials for Foam is next to impossible but Waveworld is explored by many.
I second what Andrew said. Shooting it yourself is much better and more fun!
If you absolutely need to do it digitally (why?) and photorealistically, you could do it with a liquids simulation in your 3d package (RealFlow, Blender's built in simulator, Maya's built in simulator, DPIT Effex for Cinema 4D). I don't know of a perfect way with AE.
Although, you could try Mr. Mercury.
Shooting it yourself is much better and more fun!
Yeah, who WOULDN'T look forward to the prospect of throwing white paint around against a black background?
I'm reminded of pie fights when I think about it....