I headcanon that the yellow part of troll eyes is their irises rather than their scleras because most nocturnal animals have large irises to allow for more pupil dilation. I also headcanon mutations being more frequent in the lime caste, what with our examples of limebloods being the Vantases (red blood), Callie Ohpeee (White hair and/or albinism), almost definitely Fiamet (axolotl tail), and then Idarat, the only one seemingly without any sort of obvious physical difference or mutation.