A watercolor wash is when you lay down a layer of water on your paper, then lay down a layer of color on top of the water. If you need a color that you need to mix, you should mix up the color first and then put the water down. If the color is too dark, then you can take a paper towel or a tissue and soak the water and the paint up. Part of the paint should stay on the paper in a lighter color, but if it doesn't, then you just add water to the puddle of paint on your pallet and try again.