What I really need to know is whether or not I can use artists oils or earth pigments to color the poly.

You didn't indicate what the base of your single part polyurethane is. It's either a water base or oil base. In many states oil based coatings have gotten darn hard to find.

On my last SOF I used nylon skin and coated it with ZAR single part slow dry oil based exterior grade matte finish polyurethane and I tinted it with oil based artists oils ... the process works fine ONCE you figure out how to get the color you want. I haven't tried powdered pigments in oil based poly but lots of builders do. If you're using water based poly ... I can't be of much help there because I haven't tried it ... but my hunch would be that you should stick to powdered pigments.

When using oil based artists oils to tint oil based poly there is one caveat. You are not making paint ... you are making a wash. Any particular color of artists oils are usually made using more than one type of pigment. When you make a wash the resulting color will tend toward the color of those pigments ... and it may not look like the color of the artists oil paint as it comes out of the tube or that you've mixed on a palette. An example is that many earth tone artists oils that look brown out of the tube show as a shade of orange when mixed with oil based poly. It's a little tricky to get the color you want and requires some experimentation. I've tried using established color mixing guides that artists use but concluded that they were useless for what we're trying to do by making a wash. I've been told better quality artists oils (expensive) use better quality pigments and will result in a better color match.

KenB