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.
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This depends on the pokemon's nature, not the species.
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.
Natures give your Pokemon stat changes and a personality has well not all natures change stats like Docile nature, personality just makes the Pokemon unique it also sets what kind of pokeblock or poffin the Pokemon likes.