You need to take all the excess water from your brush once you load it with
color. This is done by first wetting your brush. Dipping it in the color and
picking up the color on the brush. Now... place that brush onto a paper towel
and let the excess water flow into the paper towel. Do this a second time.
Then paint on the vellum.
While you can use watercolor on the vellum, it does leave a very powder looking
image. One for which I do not care.
What I suggest doing is what I teach in my watercolor classes. For Vellum and
Glossy cardstock:
Scribble some of your marker onto a plastic dish or onto a transparency. Then
using a small amount of water on the brush, mix it with the ink. Dab the
excess onto the paper towel.
I like the look of this much better than watercolor.
Have fun!
Carol Heppner