Let's talk about fan types. Part 10. Rainbow-type.

1. So, what is this type all about?
The idea behind this type is effectively introducing a type that is all types, or at least, most types, at once. This would allow for a species to get STAB from all types, while having only few resistances.

2.Where does this name come from?
The name comes from Rainbow Energy TCG card, which gives energy of all types. Rainbow Energy originally had its unique type symbol.

3. Does this type have an in-game source?
No, but Aurora Beam is rainbow colored, Fire Pledge and Water Pledge create a rainbow when combined, and Signal Beam is shown to be rainbow colored in the animé.

In addition, Rainbow has been a recurring theme of Pokémon, due the franchise's overall color theme.

Most notably, Ho-Oh and Xerneas are known to create rainbows. 
4. What would be the ideas behind such a type?
Predominantly multicolored and reflective Pokémon would be on this type. Outside of that, color prisms and different varieties of rainbow and reflection would work.
 
 5. What would be the type interactions of such a type?
 Effectively, this would be like a Pokémon that has all types simultaneously. Logically, for sake of balancing, immunities would likely be ignored.

In addition, Normal-type might be able to resist and hit super effectively against this type, as they are the basic type.
 6. What would be the distribution of such a type?
 Similar to Fairy, this type would be distributed to variety of Pokémon, but largely fast and offensive Pokémon would be ignored, and weak species would put forward.

 7. Future comments:
 Outside of Ho-Oh and Xerneas, several type changing species can be retconned to this type, such as Kecleon, Castform and maybe Arceus, if a Basic Plate for Normal-type were introduced.