Ahem. This would mean to really plug in some real graphics knowledge (some maths model for the “color spectrum” and things like that), area that I don't happen to know too much about. ;-)

Some fellow monks asked about related things getting back information that could also help you: RGB values and color name, color selection image map, ... (search for color, RGB, etc)

OTOH, if all that you want is "red", "green" and "blue", than I suppose you could use a combinations-like algorithm, e.g. "redish" is every tuple that has the first term numerically bigger by both others two:
#996633: 99>66 && 99 > 33 => it is "redish"