Many of BB's liquid colors are basically powdered oxides dispersed in glycerin. Like soapbuddy says, it is easier to add your powdered colorants into your base if you mix them with glycerin beforehand - otherwise you'll get clumps and specks. The liquid colors just save you that step.
For M&P, you can also dissolve your powdered colorants with a bit of rubbing alcohol before adding it to your base. To add powdered colorants to CP, I'd mix it with some glycerin or a carrier oil.
BB's website has some tips for using pigments, micas, and dyes:
http://www.brambleberry.com/Using-Colorants-W22.aspxHope that helps!