After you finish painting a room. Get a small jar with lid and fill it with the paint color for the room you painted.
Label the jar with info about color and room it was used in. Save in a cabinet or linen closet. When there is some touch up that you may need to do, you'll have the right color and small amount needed. You could also attach a small brush with rubber band to jar, thus avoiding the need to look for both brush and paint color.
It's helped me, save time and money. I have avoided having to go buy a small can of touchup paint, because I can't find the right color among all the cans of paint I have, which sometimes doesn't seem to match the color I had.