Watercolour scares and intrigues a lot of people. It's true that it can be difficult, but as with anything, you learn how to work with it. Generally speaking, you work from light to dark.....and you can't paint over your mistakes like you can with acrylic or oil.
BUT......there are ways to fix a problem area ........sometimes
I was working on a painting the other day, and realized that I had gone way too dark and blobby. Instead of filing the painting under 'G', I remembered about the Mr Clean Magic Eraser
Did you know that you can remove watercolour with Mr Clean Magic Eraser? Cut off a small piece, wet it, and rub the painting carefully. Let it dry. Re paint....carefully :)
See! It's easy!