Long ago there was an artist who did this; quite successfully I might add. However, when one was entered into and accepted into a competition, it looked so much like a gouache painting the judge was quite miffed. Basically, gouache and dry pastel are “kissing cousins”; both are opaque mediums so when you wash the pastel it looses its “sparkle”, and looks like gouache. So if you aren’t interested in competitions, and are happy with the work just go where your heart tells you to go.