Why are piano keys black and white?

The piano's white keys represent the tones of the musical alphabet: A, B, C, D, E, F, G.

The black keys represent half-steps between various notes (sharps and flats).

Together, there are 7 white keys and 5 black keys per octave, which together represent the twelve equally spaced tones of Equal Temperament.

Originally, white keys were made of ivory (white), and black keys of ebony (black). But any two contrasting colors will work. Many old keyboards present the "naturals" (lower tier) as black, and the sharps/flats (upper tier) as white!