Hi there Hilary,
Regardless of the melt & pour soap base you use (high quality or low quality, expensive or cheap), unless you purchase non-bleeding colorants, your colored chunks will bleed.
I suggest using pigments: Oxides & Ultramarines or non-bleeding micas. You can get them in powdered form at Bramble Berry , and quite economically . You just have to read and make sure each color you buy is non-bleeding. Or you can buy them in liquid form from someone like me. I hand mix all my colorants (every single one of which is non-bleeding) into a liquid form before using as it promotes even coloration, i.e., no clumping, is less time consuming but is also more expensive .
I use nothing but my own hand mixed colorants but I also get a LOT of them from Bramble Berry.
pj,
http://www.pjsoaps.com/1New.html