Summary Below:
Why do Americans spell ‘color’ and Canadians spell ‘colour’?
The American Revolution!
After winning independence from Britain, Americans felt the obvious next step was to differentiate their language.
Noah Webster urged Americans to remove unnecessary letters in words like colour, programme, and catalogue.
He published the first American dictionary (complete with new spellings) in 1806.
Canada is still part of the British Commonwealth and as such has retained many words spelled like the British.
However Canadian English spelling is unique from British English as it has been influenced by American English and Quebec French as well.