@levisan the writer chooses their dialect of English. If they randomly jump between color and colour in the same project, take points off for being sloppy or copying and pasting random text from everywhere.
Also, never say "2 miles, or 3.2 kilometers…" If the reader can't picture miles or kilometers in their head, I don't care where they're from; that's illiterate and it's not the writer's problem. Pick one measurement system and use it.
@progo I agree, I think that's the best way to do things.
It does bring up another question which I have never considered before… if you are quoting another document, do you change the spelling to match the version you're using?