The spell checker in TextEdit , for instance, drove me nuts until I discovered I could change the default dictionary to Canadian English. Doing so ended the irritating red lines under correctly spelled words.
If the wrong dictionary is the problem — and continuing with TextEdit, since it's common on both our machines — right-click on a word in a TextEdit document and choose Spelling, then in the submenu choose Spelling again. You can change the dictionary at the bottom-left.
I also found Canadian English within the International preferences in the Mac's system preferences. By choosing Edit List, I deselected English, then by scrolling down found and chose Canadian English.
That doesn't change anything in the Mac's dictionary, though. It still says that "colour" is the British spelling, rather than pointing out that "color" is correct in only one of at least 41 English-speaking countries.