Doing some touch-up painting on the exterior of the house can be challenging. For the most part, a monkey is capable of holding a brush, sticking it in a paint bucket, and dabbing it on a wall. Not that I want to compare myself to a monkey, but let's just say it's not rocket science. So whether you've had some water damage, a stucco patch, or just plain want to freshen up the appearance of a stucco wall, a good paint match is a must.
Short of imitating Picasso and mixing your own colors, you will need to bring a sample of the paint to a paint store or home center. They can take your sample and have it computer matched to get it very close IF the machine is properly calibrated. They will need a sample about the size of a quarter or larger.
Don't try to chip out a piece of stucco for this as the light will hit the rough surface and give the machine fits. Instead, walk around the exterior of your house and remove a metal surface that has been painted the same color. A box cover, such as a cable TV cover, that has been painted will work great. These will be low to the ground and will remove with a couple of screws. Another choice, although larger and heavier, is the cover to the electrical panel. I would use this as a last resort though because of it's size, and because I don't want circuit breakers exposed to the elements, or the neighbors, any more than I have to. The paint store can mix a color and then dab a little on the box cover to see how close it is...and make any adjustments.
Even if you have an original container of the paint, you can get the paint codes from it and it will match exactly right? Not usually. Over time the paint will degrade, the sun will bleach the surface, or maybe it's just that the paint Gods are frowning on your project. It will typically take a patient paint store employee to get a good color match.