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.
I think it would be a good idea to mix either TKB's pop or lake powders in some benign carrier oil, perhaps one of your base oils (a small amount taken out after measuring), mix it all up before adding lye to oils, then take a small amount of your batter near trace and mix it in with the carrier or base oil and lake/pop powder, get it mixed up good then return it to your main pot and mix it up good there too. That's the challenge with the lakes and pops, to get a nice even distribution. I haven't gotten it perfect yet but I hope the mixing with base oil before seriously mixing is probably a good idea.