I was making some chocolate clay to use for the band at the base of a cake, and it needed to be a navy blue color. Dark colors in chocolate can be problematic because you have to use a lot of food coloring to get the right color. Too much liquid color can make the chocolate act funky.
After Crystal Colors were added
I started with melted white chocolate and added oil-based blue food color to get it to a medium blue color. I then used a pot of dark blue Crystal Colors, which is a powdered color so it won't make the chocolate do anything weird if you use a lot of it.
I ended up using an entire pot of the Navy blue, then I added more oil-based candy color until I got it pretty dark.
Something else to keep in mind is that darker colors tend to get a little darker when they sit for a while, so you don't want to go too far or you'll end up with a deeper shade than you want. Add the corn syrup to it, let it set up...