Online make-up schools /courses aren't that valuable as "make-up" is hands on. So physically being at a school where instructors can give you feedback is the best option.
However I know Stan Winston offers online courses to advanced MUA's and is more focused on prosthetic lab work, and the courses are quite expensive, but they do cover colour theory in mixing colours and materials such as pigments (powder, creme), hair, latex, silicones, cold foams etc.
Joe Blasco has his MUATV online channel, but his is more of a talk show format as he interviews make-up artists from various film and tv productions throughout the decades. He does some tutorials, which are quite good... Old-school, but quite good. It's definitely make-up nerd / geek stuff, so a thorough understanding of the make-up industry is good to know to follow the geekiness.
Again online schools are not practical for newbies, but are ok for refresher tutorials for the more advanced artist. And I only know of the two mentioned above.