colours

hey all, im kinda new to blender, just got it on sunday, and i’d say iv learn’t quite alot over the last few days, but im just wondering, how do you get one area of a mesh a different colour to the other… heres an example:
http://img137.imageshack.us/img137/9869/eyemeshstartsd6.th.jpg

okay, so say i wanted the selected area blue but the rest of the sphere green, how would i go about doing that?