How to find paint that matches sculpt

I have a Gambit PF that has paint problems. His boots have some scratches. Also his black pants have some discoloration. I contacted sideshow inquiring which paint brand and color they used, but they told me that the paints are often mixed and custom. My question is how do I best find which color will match a sculpt on a touch up job? Does it come down to trial and error, while going with the color that looks the most correct?