This is the reply I got from Katherine Dewey
"If you mean alter the color before modeling, water based pigments cause some interesting reactions as the water turns to steam when baking. Heat set, polymer paints (Genesis and D'uva) are compatible but their liquidity does soften the working consistency of the clay. It's possible oil paints (including water mixable oils, without water) would also prove compatible but that the linseed and safflower oils could cause significant darkening when baked. Permanent pigments (dry or liquid) are another option for changing the color. Some alcohol based inks (Ranger makes some) have also proven compatible as pre modeling colorants.
When it comes to coloring the surface of the clay before and after baking, all of the above mentioned types of paints work well.
Hope this helps,
Katherine"
as she say's
hope this helps