Illustrator :: Create A Watercolor Effect?

Can you make a Pen Tool path (white), and then, by using the Brush Tool, color it in automatically ONLY within the path's designated area?
 
Having the ability to do this would enable me to create "digital watercolor" images with texture. I would take the white paths, and then select a color and use the Opacity Tool to make the colors look like watercolor paint by using the Brush Tool. The issue is: can I make a color texture in any path?