grey = junk
white = base
green = uncommon
blue = rare
purple = really frick'in rare (Epic)
more to come..
The color on the site is a copy of the ingame color, and is a relative guide to how "rare" the item is. Overall it doesn't mean alot, except that a blue item of your level will probably have better stats than a green item of your level, and etc on down the line.
make sense?