I think VIM is trying to use the appropriate command. If there is only one command starting with color (here colorscheme ), then it can be reduced to a point where it is not more unique. If you tried to use only col , and this fails because there is another colder command. The last sentence was mainly an assumption, so see @ZyX's comment for a real reason.
So, color and colorscheme match until there is no other command starting with color .
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