Rainbow

The Rainbow entry is not a standalone animal but a pet variant/modifier, and its listed effect says it all: it provides a bigger increase to the pet's passive. In other words, applying a Rainbow treatment turns an already-useful pet into a stronger version of itself, scaling whatever passive that pet already brings to your farm rather than adding a new mechanic.

For a farming build this is one of the highest-leverage upgrades available, precisely because it compounds with your best animals. A Rainbow version of a strong harvest-multiplier or mutation pet pushes its numbers higher, so the smart play is to reserve the upgrade for the pet you actually intend to field long-term, not a filler. Wasting it on a weak common gives you a marginally better weak common; pairing it with a Divine-tier passive is where the real value lives.

On a serious team the Rainbow tier earns its place by definition: it is the multiplier on top of your multipliers. Because its source and exact scaling are not specified in this record, treat the percentage qualitatively, but the principle holds: Rainbow your best, never your filler.