Best way to match paint on existing drywall?

I know I can do it at home depot or lowes, but I've heard this can be inexact. Has anybody used the behr paint app for iphone and had success with it? What has worked well for you?

The best way is if you happen to have the paint can with the name of the paint on it (not the brand but the color name). If you don't have that, than you can go one of two routes:

1.) If you're able to bring in a sample of the paint from the drywall that may help; although, what makes this inaccurate is not really the paint matching machines but the texture of the sample the paint is on (wood is the worse do to the grain of the wood), wear on the sample (age), and dirt. These affect the paint matching machine from getting a good read.

2.) You can take home paint fan deck cards and try to match the paint on the walls. The issue with this, however, is lighting. If you don't know the sheen of the paint on the wall and are not familiar with the different sheens of paint you may have trouble matching a fan deck card (which are usually satin in finish) to a wall that could be flat (least sheen) or semi-gloss.

Usually what I recommend if you can't find a good match or bring in a good sample of the paint is just choose a paint that is close to what you already have and paint the room fresh instead of trying to actually match the existing paint as it can be tricky to do.

It really depends on how accurate your match needs to be. If you're only touchup painting a spot in the middle of a wall then obviously you need the closest possible match to the existing paint color and sheen. If you don't have a leftover can of the paint that was used there, you'll need to cut a sample of the paint from the drywall. This is not too difficult. Use a sharp razor knife to cut a 2 inch square on a part of the wall that's free of any marks or stains. Make the cuts just deep enough to get through the layers of paint and the paper that covers the gypsum beneath (~3-4mm should do it). Then use the edge of the razor and a 1 inch putty knife or similar to gently lift this square from the wall, taking care to keep it in one piece. Be careful also not to handle the painted surface to much before you get it scanned. A sample like this should be plenty for any paint or big box store to scan and match.

If you're painting an entire wall and just need a relatively close match, then eyeballing the match from a color fandeck or similar should be sufficient. I wouldn't recommend using any app for matching paint as color reproduction on an lcd screen can be inaccurate/inconsistent.