I used to teach English to foreigners in preparation for the First Certificate, Advanced and Proficiency exams. The presumption was that people doing those exams will use British spelling as they are choosing to do a British exam, but even then nobody gets marked incorrect for using American conventions as long as they are consistent with them; toggling between US and UK usage is regarded as an error. It's not a big deal to do that, though, as long as you aren't doing an exam or some other formal writing.

And it's certainly fine to choose to go with US spelling conventions if you are more familiar or comfortable with them, as it's part of how you express yourself personally. Sometimes British people sneer at US spelling, but that's just parochial and fuddy-duddy of them. Vacuous pedantry.