Recently, I bought some Tombow dual brush pens to test out. I bought 17 colors to play around with. 4 greens, 2 browns, 2 blues, 2 grays, 1 black and a mixture of other colors. I found out that they are water soluble and work in almost any application. At first I tried them in my notebook and dug it. Then this past Saturday I decided to try them along side my watercolors.
I was impressed that they worked. You can use them as is or draw with them and then wet it for a more blended look. I was really impressed. Both paintings are done mostly with watercolor, but I used the brush pens to add a bit here and there.
I will say though that the pen can bleed easily when mixed with water. The trick I found is that if you get one area wet, to just wait till it has fully dried before getting the area next to it wet to avoid bleeding.
I should also point out that these brush pens are not expensive. A few bucks each is really reasonable for the quality of what you get.