Aniline dyes are the thing to use. There are two types. Concentrates that are supposed to be dissolved in water, and those that are supposed to be dissolved in alcohol. There are also powders and liquids, but that's a preference. Alcohol won't raise the grain after you sand, and water will, but some people think that alcohol can pose adherence problems. I've talked to epoxy manufacturers and they say it isn't an issue as long as you give it 4-5 hours to dry. I gave mine 24, just to be safe. The main brands in the States are Solar-Lux, TransTint, and one other I can't remember, but they're usually a pretty high-end finishing options that only professionals use, so you shouldn't have to worry about quality. They will fade over time, but I did some tests, and the TransTint I used doesn't seem to fade all that much. I'll try and get a photo of my test panel up sometime in the next day or two.

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