For the record, I had the same experience as Lornajane. Thanks for the helpful blog post! I came here via Google after also failing to get "color.ui" to do what it was supposed to do. I had expected setting color.ui to work based on reading the same text in the man page, but it simply doesn't.
On the other hand, setting each of the four color.* parameters individually does work. I have almost nothing else in my global gitconfig (and the system gitconfig is empty) so I find it unlikely that I've inadvertently added a config that shadows this; I rather suspect a bug in git, which may have gone undiscovered (and remains unfixed even today) because turning off color seems to be an uncommon request.