I use Stampin' Up! Markers. If you use an aqua painter and watercolor paper, you get great results.
Check out Julie's tutorial
Another thing I do is scribble on a stamp box with the marker, then pick up the color with an Aqua Painter or blender pen and then watercolor.
With either technique, you can add more color if you need to, or blend colors. And with the watercolor paper, you can "take off" color with the aqua painter.