British and American Spelling Conventions
American spellings differ only slightly from British spellings for a few words. The trend in American spelling is to drop letters that are not needed in a word, such as the 'u' in 'colour'.
One spelling convention is not more correct than the other. Just keep in mind who is going to read your writing: use American spellings for American readers, use British spellings for Canadian and other Commonwealth readers. A good dictionary will indicate both American and British spellings when there is a difference.
Canada seems to be caught in between the conventions with a tendency to drift towards American spellings of words such as 'center' and 'color' in print media.
There is an interesting article at http://www.mikefinch.com/oi/sp.htm