Hey Paul, after all these years we finally have an area of mild disagreement. :-)

: In a lot of house paints the pigment may be 20% of the volume of the paint.
: In artist's oil colors the pigment may be over 75% of the volume of the
: paint, with the better grades having more pigment.

If you are talking about solids content in paint, you are right on the money. But not all paint solids are pigment. In fact, most paint solids are not pigment, but rather finely ground titanium dioxide powder, a white opaque substance that provides "hiding power". It is very unusual to find more than 4 to 6 oz. of colorant in a gallon of paint, although the solids content can be as high as 35%. So making epoxy opaque with just colorant is a very difficult proposition. Check with your manufacturer and get their recommendation about maximum colorant ratio. In working with this problem on an industrial scale, I have found that most manufacturers limit colorants to 4 oz. per gallon or less, at the risk of voiding any warranty on the product. This does not provide much hiding power. I learned this lesson the hard way on a 35,000 sq. ft. Air Force hangar floor, but that's another story altogether.

Polyester colorants for fiberglass work fine, in epoxy they function as nonreactive diluents. They do not adversely affect the final strength of the epoxy provided they are used at recommended concentrations.

: If need be, mix half your pigment in the premeasured resin, and half in your
: premeasured hardener, getting each part smooth and well blended before
: mixing the tinted resin with the tinted hardener.

: If you mix the epoxy and hardener first and then add the pigment, then you
: cut into the potlife as you stir in the pigment.

This method helps with pot life, but suffers from a serious drawback. When stirred into unmixed resin or hardener, that liquid will adsorb onto the surface of the pigment particles, and surface tension will interfere with complete mixing. The result will be a very large number of very small pockets of incompletely mixed epoxy, and reduced strength and toughness. All epoxy manufacturers recommend complete mixing of resin and hardener before adding aggregates, colorants, etc. to avoid degrading the physical charactistics of the mix. If pot life becomes an issue, just make smaller batches. It is really difficult to control the thickness of epoxy application when it approaches the gel stage, which happens with large batches.