I have added the "regular" Wilton Food coloring to vanilla melts, and if it thickens just add a little Crisco. It works! I just use it for small amounts. (A couple Tablespoonsful of Chocolate, just a dab of Crisco). It has worked well for Chocolate transfers.
What I have found best is to buy primary colors of candy melts and mix your own colors. Also, put left over colors like blue, red, and green all together for a dark color and add black to it for outlining. I am just a hobbyist and don't want to put a lot of money in supplies. Hope this helps!