Actually show her a dictionary which it should say "colour" also. Seriously, you do not deserve to lose a point because she does not recognize other forms of spelling that vary from the Americanized one. I mean even my teachers did not force us to spell the Canadian/British way, we spelt as we felt like it.

To be honest colour > color

The first time I used the [color] tag, I typed: [colour] ^_^;