You can create focus and dimension on a final painting by glazing. This article covers the techniques every watercolorist uses to modify his basic painting.
A glaze is a very transparent layer of paint applied over an area that's already painted. Glazing has lots of possibilities. You can glaze over an area to make it less or more prominent. If you're painting a landscape and the background isn't staying far enough back, you can glaze over the whole area with a cool color, like blue, because cool colors recede. Conversely, you can glaze over the foreground with a warm color, like yellow, to make it come forward. You may find other reasons to add a glaze over something.
To glaze:
Mix enough paint so you don't have to stop midway and mix more that's a sure recipe for hard edges where you hadn't planned any.
Make the paint as transparent as possible. Glazes are usually very transparent (add more water for more transparency) unless you want to obliterate what's underneath.
Use the biggest brush you can manipulate into the space and apply the transparent glaze. Don't go over the area more than you need to avoid disturbing the layer underneath. If the layer beneath starts to move, stop or endure the change.
Some colors run when you put water on top of them, so work very quickly and use the lightest of touches so as not to disturb what lies underneath.
Some papers tolerate glazing better than others. Glazing requires a paper with less lifting ability .