The most important thing when doing epoxy grout is to make sure that you get all the epoxy off the tiles.
I just applied 2 buckets of Mapei OptiColor epoxy grout. I think that hardest part is the clean up.
The great thing about epoxy grout is that it's waterproof... and the grout color stays.
If you're about to do Mapei OptiColor epoxy grout, you may find this Mapei video helpful... I did!