I worked for a while at a combo body/repair shop off OBT in Orlando, Vince's Auto Body. The owner, Vince, was pretty impressive when it came to paint matching. He'd look it over in the shade and in the sunlight (never saw him use artificial light), then he'd disappear for a while then come back w/a touch up gun and a small rubberized magnetic sheet. He'd shoot the sheet than lay it on the panel after drying. He always seemed to get it pretty darned close. He'd do this a couple times till he was satisfied, the first time looked perfect to me but usually not to him.
He was the only paint guy I was around. He made me look at a gray metallic panel until I could see that there were actually several different colors that could be made out. Greens and blues. Or else I was hallucinating.