It may happen to stain a wall, in these cases you can clean with a sponge.
Where it is not enough, the stained part will be sanded with a lightly padded sandpaper and then retouched with a brush.
Remember that if you do not use standard colors, but colors made specifically, to keep in a glass jar a bit of color just in case of these tweaks.
If you have not retained touch-up paint, you can recreate the color by keeping the tint-meter code (machine to make colored paints. p>
Keep to paint
To preserve some touch-up paint, place it in a glass jar and not a metal one (it would tend to rust).
Then indicate the date and the room with a label, remember that over time the paint tends to separate and it too has an expiration date, it will not be good forever.