For size and colour, the other answers have covered them well.
For font choice, I can't recall where the research was from, but I read a study that showed reading large blocks of text was faster with serif fonts. But there was no real difference between serif and sans serif fonts for shorter pieces of text (like labels or headlines).
This explains why almost all novels and newspapers are printed with a serif font.
For a specific font, my vote would be for Palatino for serif and Helvetica for sans serif (purely personal opinion though).