The reason it is spelled "color" instead of "colour" in the States is because in the early 1900's there was this movement to make English easier to understand and write. I forget all the details, but we basically took all of the U's out of any OU words and some other things before giving up altogether. I think Roosevelt was behind it, which is kind of ironic.
Edit: Found an article on it.
http://history1900s.about.com/od/1900s/qt/trspelling.htm
Edit: Found an article on it.
http://history1900s.about.com/od/1900s/qt/trspelling.htm