Hi Ben,
Green is a new color for me. The way you describe the pattern of coloring, it sounds like natural markings. If the axolotl is young, its markings will develop over time, so that might be what is happening.
It really is difficult without a photo. Perhaps if you could take a picture of the axololt in the aquarium using a flash? I know that they don't like light, expecially sudden light, but if you are worried about sickness, this would be a lesser evil.
-Eva