I decided to write about this because so many clients have this problem. Even native English speakers can struggle with this.
American and British spelling are very different. British words often add a “u” after an “o” in words such as “colour”. Americans favour a “z” variation of words where British spelling favours an “s”, such as “realize/realise” and “cozy/cosy”. This isn’t an issue for me, but it does become an issue in proofreading.
Many international countries teach English, but they almost always teach American English. If you listen to a non-native English speaker, many of them will attempt to emulate an American accent - I’ve noticed this in a few people I’ve met who are native Mandarin speakers - they speak great English, but they also sound vaguely American rather than Kiwi. This results in many international students writing in American English, which is a problem when I have international students asking for help with their theses going to a New Zealand university, where British English spelling is favoured. It’s not a hard mistake to correct, but it’s probably the most common one. Several native New Zealanders also make this mistake, so I have come across English speakers doing this. I don’t blame them - certain American spellings look more “correct” to me than British spellings. However, I don’t let that affect my edits. But it’s something good to be aware of.