An interesting little aside here, and i'm sure this has more to do with application via brush vs airbrush rather than what i'm about to describe.(this particular phenomenon I haven't experienced as I'd have just touched up with the airbrush)
However, back in the days of being a mechanic (which encompassed welding, bodywork and spraying in the little backstreet specialist I worked at)
I once sprayed a panel on a customers car, all good and he was happy, a week later he returned having scuffed a different panel on a post or fence or somesuch, I had a decent chunk of the paint left and knew it was a good match and proceeded to prep the panel, mix and stir the paint thoroughly and spray it, looked fine till I got it in the light and it was noticeably, (to me), paler, I was baffled (customer was happy enough and prepared to call it done, but i'm a picky person and don't like to put my name to anything half done until a trade sprayer who used to help out when we were snowed under called by), he said, the paint wasn't mixed thoroughly enough, seen it before, I said but it was shaken to within an inch of it's life, stirred, decanted and mixed, he said, trust me, I know why, wheres the paint.
He proceeded to thoroughly mix and decanted the entire contents to a clean can, then threw in a squirt of thinner, sealed the old can and shook the heck out of it, he then decanted that to the rest of the paint, gave it me and said, there you go, that'll be perfect, re-prepped the car, mixed the paint and sure enough it was spot on.
Turns out that some of the pigment had stuck itself to the sides and base of the can and the wash out with thinner was what was needed.
Anyway, story aside, it's another little thing to think about.