Hi Erikun, and welcome to the forums.
Aside from working on a new layer, as you mentioned, there are a couple options you could look into.
The first option is using the Insta-dry feature. By turning this setting on, any new watercolor paint added to the layer will be applied over the existing paint. The downside to this, is that it won't really react at all with the old paint (almost like using a new layer), which is can be an issue if one is hoping to create a bleeding effect.
This leads me to the second option, which I happened upon during my experiments with the tool. If you select the Eraser tool and turn it's pressure down to 0%, you can use it to dry specific areas of the watercolor layer, without actually picking up any paint (if you're using a pressure sensitive pen tablet, be sure to rub lightly). Doing this, one can dry one section of a wash, while leaving the other section wet, allow for new color bleeds. I've attached a quick example of this in the screen shot below. On the bottom left side I used the normal settings for the watercolor tool, wet on wet, with no eraser use. On the bottom right, I used the eraser to dry the top part of the blue wash (the red arrow points to this area), then used the watercolor tool (default settings), to apply a stroke of red color across the blue wash. Notice that even though the color itself blends like it should, the mark it makes on the top half of the stroke reacts as though it's being applied to dry canvas, but the lower half of the stroke reacts with the still wet paint area, and blends into it.
Here are a couple of links that may be of some extra help: Link 1 Link2
Nothing is easy to the unwilling.