When I need opaque colors I've been using these:
TAP Opaque pigments
I've had very good luck with them. They are for making FRP panels such as surfboards and such, but have been working well in PR casting resin. A little goes a long way. I understand US composites has a similar product, but I have not used it.
I did experiment with some of Jacquard's textile dyes, It set fine and produced a translucent non metallic look, but that's definitely not the intended use!
I also tried printer toner salvaged from old cartridges, it does work, but needs to be mixed with something to wet it out, and the colors aren't that interesting. (well the blue is nice but not stellar) more experimenting to happen...
I did try Alumilite dye. The Florescent orange was very nice, but after sitting in a window faded very rapidly. More experimenting is in order...