Q. Can I salvage a painting, when the background is too dark?
A. The answer is yes—by putting opaque acrylic paint or gouache on top of watercolor paints. But before you try it, you should be aware that different media handle differently. Acrylics have a consistency that doesn’t allow them to be "pushed around" as easily as watercolors. Consider practicing on a separate sheet of paper if you’ve never used them. Gouache is opaque and will provide coverage, but it works just like watercolor. It can turn quickly into mud, so don’t play with it too much.
I painted In a Sacred Manner We Live using watercolor, acrylic and some gouache. I needed to break up some of the darker shapes in the background trees, but as you know, it’s difficult to paint a lighter tree over a darker one using watercolor alone, especially when your trees are all the same value. To get more covering power, I opted to go over the darks with a lighter opaque paint.
Of course, you don’t have to wait until you need to fix a problem before you try mixing and layering acrylics, gouache, or other water-soluble media with your watercolors. Many artists use the technique regularly—with great effects).
Whatever your reasons for wanting to try different media with your watercolors, make sure you use only water-soluble media—anything else may crack over time.