To preserve white areas or lines in watermedia, try resists. If you use wax-based colored pencils, like Prismacolor, watercolor slides right off. Draw with a white crayon or drag candle or paraffin wax across rough paper to achieve texture. You'll love the sparkle effect, but use sparingly. If you want to remove the wax, lay the painting face down on brown paper and run a warm iron over the back. The wax will transfer to the brown paper. For a fine line, place a sheet of wax paper (waxy side down) on your paper and draw on the wax paper with a sharp pencil, using enough pressure to transfer the wax line without cutting through the paper. Remember, when using clear or white wax resist, you can't see the lines until you lay the wash. So be sure you know where you want your lines. You can also use liquid frisket, masking tape, oil pastel as a watercolor resist.
Resists and other experimental techniques: The New Creative Artist pp. 139-155.