how are old paint colors figured out ?

BYK- GARDNER makes a handy little scanner that will scan an area of the color. Problem is that colors tend to fade over time. If single stage not so bad. Two stage base coat clear coat, the clear will yellow over time. Nature of the beast. But, the good news is a GOOD shop and a GOOD color match guy can get it so darn close only a trained/experienced eye can tell the difference. Hope this helps.