Prismacolor pencil on manilla folder. That stuff works great! This is a good example of what's called the "burnishing effect", where colors are blended together and "built" upon each other to define the overall colors. When this process starts, it looks like crayon in a kid's coloring book. For instance, if the overall color is "yellow", create your base with an even spread of yellow in that entire area then build on it with maybe yellow-orange and work your way into the darker colors or lowlights. Top it off with white or light colored streaks and BOOM you've got a cool-looking area that has volume.