If you don’t know the color of the paint it will be tough.

You could try to get a sample off the wall (maybe behind a picture or somewhere inconspicuous) and take it to a paint store or Big Box store and have them match it, but it still probably won’t be perfect. If you do this, pay attention to the sheen. If the wall is glossy and your touch up is flat, it will stand out.

If you can’t get a sample you might be able to match it by eye if you get a “fan-dex” from a paint store—the thing with all the paint colors from a certain brand in it. There are some tricks to matching by eye, but you can do it with practice.

If you can touch up a larger portion that goes all the way to the edge of a wall it will be much less obvious.