Hi all,
I had posted a question on this topic a few weeks ago and wanted to report my experience.
I was looking for a way to "color" the inside of the kayak without using a paint topcoat, and landed on adding pigment to epoxy.
I didn't want to add pigment when laying the fiberglass- I wasn't so worried about the pigment effecting the bond, but thought it could make it difficult to confirm a tight layup. I have my hands full using 6oz glass on a concave surface as it is. I used System Three black pigment. Instructions say not to exceed 10% pigment to epoxy, though I had read some ominous product reviews (always need to take those with a grain of salt) cautioning against approaching that ratio- suggesting that it affected the strength of the epoxy. Numerous folks reported that only a tiny amount is needed.
I ended up using approximately 3/4oz of the pigment to 24oz of epoxy. In the pot and on the stir stick, the mixture seems utterly opaque.
I rolled on the epoxy- probably a bit thicker than I would have if my only intention was to fill the weave- and was impressed with the coverage. I will definitely do a second thin fill coat to make it completely opaque. I don't feel that adding more pigment would have made much difference in this regard, and a single thicker coat poses problems, too. To another time I'd count on two thin coats (at this concentration) for good coverage.
This is far more opaque than graphite, and without altering the properties of the epoxy.

Patrick