Energy » Rainbow Energy
If Mewtwo "Energy Absorbs" a Rainbow Energy and then Venusaur "Energy Trans" it, does the receiving Pokémon take damage?
Nope, it only does [places] the damage [counter] when you play the card from your hand.
Source: WotC Chat (2000-06-22)
Energy » Rainbow Energy
Can the First Movie promo Pikachu use Recharge to get a Rainbow Energy from the deck, or does it have to be a basic Lightning Energy card?
That would not work. Rainbow Energy only counts as all-basic energy types WHILE IN PLAY. This is not the case when it is in the deck.
Source: WotC Chat (2000-04-27)
Energy » Rainbow Energy
Can you have more then 4 Rainbow Energy cards in a deck, since it changes into a basic Energy?
No, since it's not a basic Energy card you can only use 4 in a deck.
Source: WotC Chat (2000-04-13)