I just got this from manchesteronline:
What wristband colours mean and the causes they support:
Yellow: Lance Armstrong or supporting US troops;
Blue: Anti-bullying, anti-Bush vote, tsunami relief, prostate cancer;
Pink: Breast cancer;
Red: Heart-disease, vote Bush, anti-tobacco, HIV;
Purple: Cystic fibrosis, lupus (also orange), domestic violence;
White: Jesus Loves Me, right to life;
Orange: Asperger's, self-harm;
Grey: Diabetes, brain cancer;
Black & white: Racism in football;
Green: Ecology, leukaemia, organ donor;
Gold: Childhood cancer;
and Black: Mourning, melanoma.
Not an exclusive list.
Generally they cost around £1 each but some are hard to find and many cost more than that just for p&p.
You can find lots on ebay but be aware that a) the money you pay probably won't go to the charity and b) they will clobber you for p&p even worse than the charities do - eg £3.95 (for a tiny light plastic band ). And the money goes in the vendor's pocket.