Leave small amounts of each item used on a job for the client. This might include a bundle of shingles, a roll of wallpaper, a few pieces of siding, a can of paint, or a box of floor tiles, for example. If clients need to patch or replace damaged material, they will have an exact match, and in the future if they add or redo something, they will have a sample and the product information to make the best possible match. Leave the items in a cool dry location.
EOB, Customer Notes
One reader wisely suggested attaching a business card to the items you leave, as a marketing tool.