I have to share this with you. I have discovered a great way of erasing watercolor to fix mistakes. An electric toothbrush! It is cheap (Colgate 6 bucks) and you can accurately pinpoint what area of pigment you want to remove to regain white paper since the spinning of the bristles stays in a desginated area instead of the longer strokes you need when doing it with a sponge or an old oil brush. Besides you control the pressure, so this tool damages the paper less by only allowing minimum friction. Make sure you get the one that has the one spinning head.
Also in case you don't want to erase but add onto your painting, pastels and watercolor look very similar when placed on the same paper. If you use Arches cold pressed you will see how compatible those two mediums are. I prefer pastels for working over a watercolor rather than the other mediums such as acrylics, Gouache, or Chinese white. These latter mediums tend to produce a shine resulting to a patched up watercolor  . However soft chalk pastels gives you a totally identical look to watercolor and you cant even tell you added it. Just make sure that when you add pastel to your painting push the powdery pigment into the grooves of the paper with your finger. It will look like you painted it in watercolor from the beginning. Pastels also are great to soften hard edges in your watercolor in case you didn't do while painting. This results in a perfect wet on wet technique.