Pour some warm water into a bucket and mix in a squirt of dish soap.
Dip a rag into the soapy water and clean a small section of the wall with the paint color you want to match. Do this in an inconspicuous area, such as behind a couch. Rinse the spot with a dampened rag and allow to dry.
Cut a small, square patch from the wall with the utility knife. Just score the drywall and peel up a patch about 2 square inches.
Take the color patch to a home improvement or paint store that has a spectrophotometer. An associate will put it into the device, which will analyze the color and provide an exact match.