can you not just take a paint chip to sherwan williams or equivalent and have them match it? That's what we did when we repainted our house just to make sure and we had done the previous paint as well.
A chip as in a sample of the drywall... that I've found to be much more of a mixed bag. My last "matched" paint was nowhere close once the sun was shining on it.
Each major line of paints has slightly different colors, so figuring out of it was Sherwin Williams, Behr, Valspar, etc let me identify the right color collection and narrow things down enough for me to guess what he was using.