I've rethought a bit about this card. It's not as overpowered as we may think.
You think it's overpowered because the colorless Nymph produces an infinite supply of alchemy cards, right? The other Nymphs already do that. If you think Rainbow Nymph + FFQ = infinite Unstable Gas is overpowered, then why isn't Blue Nymph overpowered? The Rainbow Nymph is actually weaker than other specialized Nymphs, because it adds the card to your hand, and it requires you to cast it; AND it requires you to sacrifice a creature. Other Nymphs have neither of those two restrictions.
Thinking about that, maybe Rainbow Nymph's ability should cost 3 random, or even less.