I've worked with some 2-part epoxies.
Of course you can use glitters, and there are liquid colorants made specifically for epoxy, but I use dry pigment powders.
I measure the non-hardening resin first, then mix the pigment powder into that non-hardening resin. Then measure the hardening resin and stir it into the mix and then use the resin.
I HAVE, however, used X2 colorant in those same resins with 100% success.
Many of my pigment powders come from here...
https://justpigments.com...but there are other craft pigment powders available elsewhere.