How to Fix Mistakes and Remove Errors in a Watercolor Painting

1. Make a decision whether it really is a mistake or whether it adds an unexpected quality to the painting. Consider letting the painting go in its own direction rather than forcing your preconceived thoughts on it.
2. If you do want to get rid of something, and you've only just put down the paint, try stimulating it out with a piece of paper towel, absorbent cloth or sponge. Don't rub or press tough as you'll push the paint into the paper. Rather, strike very gently, using the paper towel's ability to absorb liquid (paint). A dry brush will also work -- clean it on a cloth to remove the moisture in it, then put the very tip onto the wet paint; the brush hairs will absorb the paint.
3. If the paint has begun to dry, leave it to dry completely. This may appear a counter-intuitive, but by leaving it to dry the paint will be on the surface of the paper and painting. Then take a clean, slightly damp brush (not one that's very wet) and gently brush on the area you want to remove, functioning into an adjacent bit of the painting, then use paper towel to lift the paint.
4. If the paint doesn't come off in one go, don't scrub with the brush on the paper as you're possible to damage it. Quite, leave the watercolor to dry and then repeat the process.
5. If you believe the whole painting is a disaster but don't want to waste the sheet of paper, submerge it in a bowl of clean water to soak out as much paint as possible. Enlarge the piece of paper again. It'll probably keep a shadow of your previous painting, so it'll be better suited to a dark or colorful scene than a light, bright one.