When I started high school my ninth grade English teacher gave my class a spelling test of simple words, most of which coincidentally had multiple accepted spellings. Color was one of those words, but I wanted to be a smartass so the entire year I used alternative spellings for everything I could, especially colour.
Needless to say I didn't get very good grades that year but I did pass. I ended up getting a lecture that I needed to do everything 'the American way.' Come to think of it I probably shouldn't have been correcting my teacher's grammar, but I digress.
Long story short, I switch back and forth between the two.