Pokémon must retain their colors throughout the battle. For example, Charizard-Mega-X is banned, because it changes from Orange to Black.
You must carry a full team of 3.
Shiny Pokémon may be used if their base forms share a color with the rest of the team.
Visit https://bulbapedia.bulbagarden.net/wiki/List_of_Pok%C3%A9mon_by_color for a list of Pokémon by color.