I'm English, so colour.
As a few others have said, it was an American attempt at simplifying the English language *resists temptation to have a cheap dig at the Americans*
I don't think it's a huge deal though, if I ever moved to America I'd spell it color because I'd look silly if I didn't. You adapt to the culture around you.
I'm surprised by the results of the poll, both by the amount of people who spell it colour, and the amount of people who spell it 'Charizard'
A little FYI, Chrome is picking up the American spelling of colour as wrong, I guess it depends which dictionary your browser is using.