www.pcb-creation.fr/ makes the colors already emlusified in cocoa butter (I find that easy to use and you can air brush with them). You can buy them thru many gourmet sources, I get mine thru classic gourmet. You pop them in the micro then use, very easy. There's also oil based colors, that work in chocolate.
Your method is important and correct. Apply the color to the exterior, then pour the bulk chocolate over it.