I have better luck with liquid candy color - it is oil based and is specifically formulated to color chocolate. I also have powdered candy color, but I find that can cause specks of color in the chocolate, especially violet and red but I have no idea why the speckling occurs only those two!
You can buy candy color from various sources; craft stores usually have tiny pots of candy color - I get mine from Pfeil and Holing out of NYC. http://www.cakedeco.com and look up colors. P&F;sells mainly to businesses; there have to be a lot of places that sell to consumers. Maybe even King Arthur flour has it?