Soft Pastels are interesting media to work with. It can also be fun for you can use your own fingers to apply the color or you can use a brush or cotton balls.Just outline the drawing with your chosen color and smudge it with your finger, brush or cotton. You can achieve a soft, hazy look with this just like the one in the picture. Plus, you can actually erase smudges using eraser.
For a much lighter effect, you can mix a small amount of soft pastel with baby powder and apply it with cotton balls. This technique is usually used for gradient backgrounds. Lastly, enjoy your work! You can keep it in your portfolio folder or hang it on your wall. There are still a lot of ways to use coloring materials in order to give life to your drawings. Just don't be afraid to explore.