Difficulty: Moderately Easy
Things You’ll Need:
Step1
Whether you are painting over repair or just "touching up" over grimy or worn areas, the best solution is to get the paint color as close as you can, then paint the entire wall from corner to corner.
Step2
If there is something you can take off the wall to have the color matched at the paint store, this is the best solution. A heat register or painted switch plate cover, perhaps.
Step3
If there's nothing you can take off the wall, get as many paint swatches as possible and hold them up to get the closest match. Do this in natural light, if possible.
Step4
Say the wall you're trying to touch up is really high and it's not practical to paint the entire thing. Use a roller instead of a brush to apply the paint, it will blend in better. Realize it will be impossible to do a perfectly seamless match, but if you get close enough most people won't notice.