My personal favorite is the dry pigment from K&G.......I dislike the liquid types......I've found they sometimes cause curing issues. I've also seen some of the really el-cheapo stuff change colors upon curing.
I use Acraglas epoxy for my handles, and the only dye that I mix with it is the black Acraglas stuff that Brownells sells. It takes incredibly little dye (just a speck on the end of a toothpick) to dye a hidden-tang mix jet black. Cool stuff.
The power pigments from hobby lobby work great and cost very little. you can get any color and works on epoxy and the gel super glue as well. You just have to use it quick if you color super glue.
I have been using it for 20 years and I bought a 5.00 jar of each red, black, blue, green then and still have a lot.
Can't go wrong and easy to get.
I have to say, I've only ever used http://liquidglassepoxyresin.com/. However, I've created many projects and seem to have the most success with dry pigments. It doesn't take a lot to add color and make the project look outstanding.