What color makes up the highest percentage of LEGO elements produced since ABS plastic was introduced in 1963?
Peeron.com has a colour table that lists how many pieces of each colour you would have if you had one of every set in their database.
Obviously their database is not complete, it doesn't relate to when ABS was introduced (but most of the pieces produced in all time will have been since then), and some sets have been produced in higher numbers, so that's not an incredibly reliable figure, but seeing it has more than double the closest rival, I think it's safe to assume that Black's the winnner.
I would think that it is partially due to the fact that technic pins come in black, and there can be hundreds of those in sets.
Incidently, the top 5 are: