During my first year or two in a paint department, we had two guys come in with a used five-gallon paint bucket wanting more of it. Typically there is a label with the color's name on top, if not we can take some of the old paint and scan it with the computer eye though it's not as accurate. These guys had no lid and for some reason their bucket was crazy CLEAN. There was nothing I could use to get their color. The guy looks at me and says "why can't you do it, this is you guy's paint!" even though I had just explained why to him. That's one of the moments you hate working with the general public. A paint mixer is not a magician.