For an entire wall
The paint sample fan you choose from and the color that ended up on your wall may have slight differences you have no way to prepare for. And after you’ve applied it you realize things are different than your original plan. The good news, however, is that you don’t have to wait very long to paint over it. According to professional painter Wayne Rushing, latex interior paint needs about 2-3 hours to dry. Still, his recommendation is to wait a full night before you do anything.
If you want to paint a light color over a dark one, you’ll need a coat of primer. Rushing recommends trying a small spot as a test. It is likely that you won’t need more than one coat of primer and two coats of the new paint. Rushing’s pro tip to save time is to get his primer tinted to match the new paint. This might save a coat of the new paint you apply.
For a badly painted trim
Did you go for a less-ambitious project and only painted trims around the room? Cool. Are you having second thoughts about your choice of color? Not so cool, but not the worst thing that could happen.
It is likely that you used oil-based paint. This means that you will need oil-based paint to cover it. If you used a different kind of paint, you have some sanding to do. When you’re done sanding and removing the stained trim, you can prime and paint it.