I've just noticed something interesting:
One apparently widely supported theory about who is the half blood prince says it's Godric Gryffindor. Now, choosing the colours of the four houses, JKR seems to have gone with the traditional meaning of colour, such as the fact that red is associated with passion and courage while blue (Ravenclaw) symbolizes wisdom. Now, the colours of Gryffindor, red and gold, do not only stand for courage but both are traditionally a symbol of royalty. In china, for instance, gold was the royal color used by the emperors, representing power and authority. Under the penalty of death, no Chinese person other than the emperor was permitted to be clothed in any shade of yellow or gold. In the old western monarchies however, the colour that traditionally stands for royalty is red.
This might point out towards GG being the Half Blood Prince.
Another thing is the colour green, the eye colour Harry inherited from his mother. Traditionally, it stands for harmony and unconditional love.
There's however another side, the 'green from envy'-side. JKR somehow seems to connect green with evil: It's the colour of Slytherin (together with silver, the counterpart of gold), of snakes, basilisks, Lucius Malfoy seems to favour it, and the light of the unforgivable curses is green.
Maybe something of this explains why Harry's green eyes are so important? I dunno...I just wanted to mention it.
Slytherin Green Grin