I found this link, which indicates that the normal range for color learning would be 18-36 months:

http://www.babycenter.com/404_when-will-my-child-know-his-colors_6717.bc

I remember being puzzled by this as well, because DS learned shapes very early (12-18 mos), but seemed to not recognize colors as well. He learned them eventually (can't remember the age). I suspect it's developmental...