Well you needed a contrast between the keys and you also needed something durable. So with Ivory no explanation needed and ebony being the hardest wood known. note this was long before plastic came about.
If they were red and yellow, Paul McCartney's song would make no sense. :P
used to be made out of actual ebony and ivory
Because the keys were originally made from ivory and ebony wood DUH.
Good point though, there is no reason why they cannot be any colour nowadays. Im surprised someone has not done this already.
Yeah, so many different combinations that would work really well and it would be a nice change visually.