I've used both pop and lake. They have been stable but I have had minor problems with specks, small concentrations of colorant due to insufficient mixing. I add color by removing a small portion of soap batter, coloring that, then returning it to the mixture. I intend to focus a greater degree on thoroughly mixing the colorant into the sample, and may try pre-mixing the colorant with oil or something.
Liquid colorants are often easier to use. I've mixed Select Shades right out of the concentrated containers, counting drops into my soap pot while I'm mixing.