Option 1: Remove a sample from your wall & have it color-matched. This is reasonably easy to do if your walls are drywall. You can use a craft knife to very gently cut through just the paper layer of the drywall.
Option 2: Use an electronic color matcher. It used to be true that to have a paint color analyzed, you had to head into a paint store with a spectrophotometer.
Option 3: Use a paint matching app Lots of paint companies, including Behr, Valspar and Sherwin-Williams, now have mobile apps to help you color match paint. The apps are all different, but the general concept is the same for each of them.