Chocolate colors (oil based) have advantage which is can be use not only in chocolate, but also in icing and batter. Where as in paste food colors or icing colors (water based) can not be used into melted chocolate.

Powder chocolate colors and paste are no difference, use them in a same way. Pour them into melted white chocolate, stir it well. If they are not dark enough or bright enough, add a little bit more of the colors. Powder chocolate colors have advantage than paste chocolate colors which is powder colors can be brushed on the surface of icings or hardened white chocolate.