This depends on the pokemon's nature, not the species.
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That will depend on its nature not what kind of pokemon it is. Test out each color until the dialogue says that the pikachu liked it.
Dingle Berries
It depends on its nature, or just give what color it is. A brown pokeblock would do
Pikachu is an electric Chincilla. Many think it is a mouse, but the body and tail are too big for a mouse. --No its not, Pikachu is an electric mouse. Pidgeot is a pidgeon but is also too big for its size. If you have a Pokemon game, go the the Pokedex and you will see that Pikachu is described as an electric mouse.
um, that game is kind of illegal so...