Pastels are a distinctive way of accenting a watercolor after the painting has been completed and dried. One method of adding detail is through the use of small, distinct lines, which can be part of the overall outline or create extra highlights. You can use pastel sticks as a drawing tool to add lines to leaves, tree trunks, water, buildings, clouds and rocks. For this technique to work, render the original watercolor with as much color and richness as possible.
In this scenario, again you should execute a watercolor painting that can stand on its own. After the watercolor paint has dried, you can add solid areas of pastel color to create shadows and accent the highlights.
If you are a novice painter, this exercise can help you understand the difference between pure and mixed color, for when painting with watercolors, it can be difficult to create saturated color. When you add more color with a soft pastel, however, the natural forms can take on a deeper hue that enriches the painting. Eventually you will be able to create the stronger color by using only watercolor paint, if you so desire.
Instead of waiting until the watercolor dries, apply pastels while the paint is still wet. When working with this method, you can either complete the entire watercolor and then wet each area where you wish to add the pastel effect, or you can do one small section at a time, mixing the pastel with the wet paint as you go. This second method definitely is considered the more experimental and unpredictable. You might try this method if you wish to find new and interesting abstract designs in your watercolor paintings.