Going by advices given to me in the past, you would want to save rainbows for when you want to skill awaken a certain unit. To do this, you would want to go to the Japanese wiki for Aigis and look up what your units get after skill awakening. If you want to max skill Minerva, you can look up what she gets after skill awakening.
Generally, you would want to spend rainbows on black or platinum units. Spending it on Leeanne would be a waste as it is easy to get many copies of her.