Got me curious so I have been having a browse.. cant find anything at all about adding pigment to polymer clays.. but plenty on how to blend clays.. most reccomend a complex regime of cutting and stacking, then rolling using an old pasta machine! - and it is all trial and error..
Coincidentally I looked up pigments and their uses,
http://en.wikipedia.org/wiki/Pigments and realised something which will be important to anyone hoping to take "white" clay and colour it.
White polymer clay has pigment in it (titanium or zinc it seems are the most common..), so no matter how much you blend, you will end up with a combination.. so unless the colour you want to achive is blendable from pigmeted clay, you will never get a true result unless you find unpigmented clay as a base!
You can get artists oil paint pigments - some still prefer to make their own rather than buy a branded paint so maybe you could get some pigments and have a plitter as my mother would have said!
