Water based food coloring will work with epoxy and polyester resin, for urethanes it will create foaming...
For translucent dyes, it's one of the only Castin' Craft products I will use, Michael's and Hobby Lobby both carry Castin' Craft translucent dyes that will work fine with clear urethane... They are not that concentrated and weak drop for drop with Smooth On or Alumilite tints but you can get them in a pinch... Hobby Lobby also carries Alumilite pigments in their little Alumilite display...