"What colour are her eyes?"
Translation:Beth yw lliw ei llygaid hi?
It is, but remember that does leave some sentences a bit ambiguous as "ei" means both "his" or "hers" and the only way to tell is if you put the pronoun in after or if the following word mutates softly or aspirately.
As EllisV says, it is optional here since llygaid has not mutated, and so it must mean 'her eyes'. The sentence has now been updated, although the change may take a little while to filter through.