Personally I'm not bothered by British/Aussie/NZ spellings for words in fic, even if the viewpoint character is American. I mean, it's true that John probably wouldn't be thinking "colour" but I don't mind reading it. I tend to think of that as being a function of the writer, not the characters; I'm used to books written and published in Britain using the British spellings (understandably enough) even if they're not taking place *in* Britain, if that makes any sense...