Hello, Google is smart enough to figure out that both spellings mean the same thing and will give equal importance to both. I don’t think there is much to worry about. However, just to connect with both shopper groups, you can create a mix of both.
Google doesn’t really care, so the real question is, does your audience?… For example, if your niche is Photoshop and color (but of course, *colour*) correction, both sides of the pond won’t care one way or the other. Firearms, on the other hand, is a topic where British “accent” automatically lowers your authority by at least 50 points.