Awww, I know what you mean, I often do the same with hair, particularly if I don't even have that hair mesh in game/can't find the whole set - I hate not having at least one of each natural of a mesh... (I secretly wish creators would tooltip their creations better so you can search for them)....
Moonbeam's eyes are just a light purple/lavender color, but if you happen to have nilou's Last Day Of Magic eyes in game (available at both gos and her lj) they are actually VERY similar eyes and i know nilou's set has a light purple as well....
that being said, i'm sure she'd look good with any eye color, and i don't have a policy disallowing changing her eye color completely if you really want her :P