Rainbow Energy constantly counts as
all Energy Types. It only provides one unit of Energy at a time, however. If one turn it is meeting a [M] Energy requirement but the next you need it to fill a [W] requirement, that's perfectly fine. Historically, there have been some attacks that exploited the nature of
Rainbow Energy (and similar cards). While it only provides one unit of Energy for the purpose of paying for attacks, some attacks have secondary effects based on what Type of Energy is attached.
For example, there is
Mew (
Southern Islands 1/18). It has the following attack:
[P] Rainbow Wave: Choose a type of Energy other than [C] attached to Mew. This attack does 20 damage to each of your opponent’s Pokémon of that type (including Benched Pokémon). Don’t apply Weakness and Resistance.
This led to the following ruling, which I has never been overruled in all these years and thus is still applicable:
Q. If I just have a Rainbow Energy on Southern Islands Mew, will that pay for both the P attack cost and also allow me to choose another color of Pokemon to do the damage to?
A. Yes. But you only do damage to one type of Pokemon. Rainbow counts as all types but only provides one energy. You choose what type of Pokemon you want to do damage to when you use this attack with Rainbow Energy attached. (Aug 9, 2001 WotC Chat, Q233 & Q286 & Q345)
Why bring this up? It helps demonstrates the point: even though one is using
Rainbow Energy to meet a [P] requirement for the attack, even during
the same turn, in fact in the middle of an attack, the player can still reference any of the other Types for the effect of Rainbow Wave. Some supplemental examples with slightly more recent cards
Energy Signal (Roserade - Unleashed)
Q. What happens if I attach a Rainbow Energy to Roserade with the "Energy Signal" Poke-POWER? Do I get to use both effects?
A. If the Energy card you attached provides all types (for example, Rainbow Energy), then both effects would trigger. Energy Signal would both Confuse and Poison the Defending Pokemon. (Sep 30, 2010 PUI Rules Team)
Rainbow Energy
Q. If I have an attack that says to discard a Fire Energy and a Water Energy, may I discard a single Rainbow Energy card or Multi Energy card to satisfy that requirement?
A. No, even though Rainbow and Multi provide all energy types simultaneously, they only provide one unit of energy. You can discard it for either the Fire or the Water, but you would still need to discard at least one additional card of the required type. (Dec 11, 2008 PUI Rules Team)
Always counting as all Types of Energy while in play does backfire on occasion. I cannot find the ruling right now (almost out of time),
but if you have a
Rainbow Energy and an opponent's attack, Ability, etc. refers to a particular Energy Type, it can hit
Rainbow Energy. So if
Psyduck (
BW: Boundaries Crossed 33/149) uses its "Firefighting" attack on a Pokémon with a
Rainbow Energy attached, unless the effect of
Rainbow Energy is being negated then Firefighting will discard it.