John in NM
I've had good luck with pre-tinted products - fortunately I'm making black parts so pre-tinted is available. In this case I'm using it for casting rather than filling, not sure it would work well as a filler - stuff goes off fast, 3-5 minutes. Makes a fantastic casting resin though.

The first stuff I tried did require a pigment, but the vendor supplied a paste pigment that worked well - no idea what the fluid in the paste was, something that boiled higher than the cure temp of the resin. Before the vendor started carrying black resin, I was going to try some lamp black powder. The trouble with dry pigments is that they can introduce additional air to the mix, but since I'm pulling a vacuum before pouring that was less of a concern.

I would guess that dry pigments would cause fewer curing problems, being basically inert. With paste or fluid pigments, I try to keep it under 5% by weight.