Answer

Touching up an existing painted surface can be challenging - professional painters wrestle with this constantly. Ideally, use paint from the same can that was used originally, but reduce it about 10% to 15% with the reducer recommended on the can. If you are touching up walls on which the paint was applied with a roller, use a small trim roller. If the paint was brushed on, use a brush.

Apply a small amount of the touch-up paint and "feather" the edges, starting at the outside edge of the touch-up area into the center of the area. "Feathering" entails drawing the brush across the area outside of the touch-up onto the new paint to create a transition that diminishes the appearance of the touch-up. If the surface had to be patched, use a primer sealer.  Try to prime and paint to a natural break. Please note, though, that sometimes repaired areas may be noticeable. In this event, painting the entire wall may be the best option.

Reference: Sherwin Williams