Have to remember North Americans speak American-English, not actual English. There's something about them having to make their own language in their constitution (or so I hear - it's not something I've checked) so that's why there's several spelling crazinesses (as well as other reasons, I'm sure).
So, when talking about English - yes, "colour", "theatre" and "centre" are correct.
If you're talking American-English, then no - they'd be wrong.
(Note - I may have foolishly used "North American" when I should have used "US citizens" or something similiar.
I for one don't want to offend Canadians - especially as they still celebrate Victoria's birthday - very Steampunk
.)