Having lived in the US for a few years (but now back in the UK) I get confused by many of the UK/US spelling differences. Not the -ise/-ize or colo[u]r type stuff, but things like "tyre/tire", "centre/center", "litre/liter", etc. It'll eventually sort itself out I suppose.
Pity us Canadians, then. We use a pseudorandom mixture of the two. There are, apparently, rules, but they change weekly and there is something in the Statute of Westminster that forbids any discussion of them. No two Canadian English spelling dictionaries agree on anything, nor do any of the style guides. We just carry on and hope that no more than 30% of our Canadian readers think we're idiots on any given day.
Someone with one watch always knows the time, but someone with two watches is never quite sure…
Bear in mind that while homographs in American English, "metre" and "meter" are different things.