To the best of my knowledge, there is no automatic spelling and grammar engine on the GMAT essay scoring. It's done by individual readers, and therefore as long as you're using British spelling consistently, they'll probably see that it's not a mistake, but just a different style. You'd only run into real problems if you switched back and forth, e.g., "colour" in one paragraph and "color" in another.

I don't know the answer, but one of the markers for your essay is a computer program.

I hope the computer and human marker take into consideration different English spellings. As a precaution, I only used American spellings. This wasn't as difficult for me because here in Canada most words are spelled the same and we are used to seeing the American spelling for things.

That being said, the system is not designed to penalize the occasion spelling gaff.