Pigments - Oxides, Mica

Dry colour pigments suitable for cosmetic applications. These pigments can be used to 'paint' soap bars too. Usually mica is used to paint soap, with isopropyl or 'rubbing' alcohol mixed, to make a runny paste. 

If you are looking for liquid soap 'paints', any non toxic acrylic water based paint, ie kindergarden finger paint or 'school use' paint is safe to use. As long as it is non-toxic and safe for children, it's safe to use on soap :)