Well, his dermatologist (and make-up artist) have released various statements, so unless they're both lying, I'll say they're the closest source anyone can have. They both confirm that Michael was diagnosed with Vitiligo and Discoid Lupus in 1984, which sounds accurate to me. Many photos of him during that time show he is clearly covering his white blotches with make-up (sometimes not very successfully, lol) and and his cheeks begin to always seem to have a red tint to them. You’ll see it all over his face and even his neck.
Same with the lip color, it started as a way to conceal the discoloration on his lips, if you’ll notice the discoloration on the outline of the lips in the first picture, and some discoloration on his nose in others.
Michael’s makeup artist for over 20 years, Karen Faye comments:
“It started happening relatively early, he even was trying to hide it from me..he tried to hide it for quite awhile. He’d always try to cover it with makeup and even out his skin tone until it got so extensive. It’s all over his body. We were always trying to hide it and cover it for the longest time until he just had to tell Oprah and tell the world, ‘Listen I’m not trying to be white, I have a skin disease.’ In the beginning I tried to cover the light spots to match the darker part of his skin, but then it became so extensive that we had to go with the lighter part of his skin because his whole body was reacting…he’d have to be in complete full body makeup, every inch of his body. So it was easier to make the transition to him being to the lighter shade that he is.”
I'll have to go find the video on YouTube where she says this, it was in the "Living with Michael Jackson" Martin Bashir documentary. The extra footage version.