Matching paint means never having to say... OOPS!
Matching a paint color is as easy as putting a hole in the wall...
The assumption here is that you need to repair the wall, or maybe the paint has been stained somehow. Oh... and you don't have an old can of the paint laying around in the basement!
You could borrow or buy a set of paint chips (left) from your local paint store, but I have found a quality paint store will do a better matching job than most of us mere mortals can... if you provide them with a small paint sample from the wall to match against.
If your walls are drywall, simply cut out a section of the paper only with a utility knife. Cut an oval-shaped, 1"x 2" slot in the wall, piercing only through the surface of the paper... an eighth of an inch deep is plenty. Use the knife to lift a corner of the paper, and then peel it from the wall. Take this sample to the paint store for matching. The cleaner the cut, the easier the patching.
You can also cut out a small section of plaster, though it is somewhat messier.
Don't do touchups in high traffic areas or over repairs... Paint the entire wall instead!
If the walls were painted more than a year or so ago, the walls will not touch up well (the exception being with stock off-whites from the same manufacturer... in other words an exact color match). The effective gloss (or sheen) of paints from different manufacturers (yes... even flat paints vary in "flatness") will make a touchup show even in the color match is perfect! Your walls will look like they have some sort of disease!! This is especially true in high traffic, fingerprints-on-the-walls type areas!
The solution? Get a quart of closely matching paint and just paint that wall, corner to corner, being careful not to get any of the new paint on an adjacent wall. You will be amazed how even slightly different colors will blend well at a corner.
Doubt me? Look at the graphic to the right. The two walls meeting at the corner are exactly the same color. But because of shadows (based on the locations of the light sources), the left wall looks shades lighter than the right wall! This is why an exact color match isn't necessary as long as you are somewhat close in color and you paint the corner neatly. No one (but your hairdresser) will know for sure that you didn't use the same color! Plus the bonus... you don't have to repaint the entire room!
If you must touchup new or old work...don't ever just brush on the paint. Feather it in!!
Even if your paint is a perfect match, if you just brush it on, it will appear to be more glossy than the rest of the wall! Instead do this: