Deb Ammerman shared a fascinating website on her blog (see sidebar) where one can go and "scribble" in a fascinating program. This is the link: http://www.zefrank.com/scribbler/scribblertoo/
I'm afraid it could be addictive. You can save your results and go from there. You can change the canvas size, brush aspects and colors. I just tried the basics but saved one and altered it with underlying color. What fun...and maybe something more.
It seems I just can't resist. Here is another with underlying color added.