This technique is for using Prismacolor pencils. Take one pencil color, solid fill the space, than add another color over top of (either light or firm pressure in the hand and pencil). You may use several colors if you wish but each time become a little more heavy-handed, pushing the pencil pigment into the previous layer. Remember to experiment on another paper before attempting to do so on your final drawing because if you use a wrong color it is very hard, if not impossible to correct. Also test the colors together beforehand due to the combinations of different colors altering to create a totally new color. As you burnish the colors together, the result shall look more waxy or plastic, which in turn makes the drawing have the appearance of a painting rather than a sketch drawing. Should you like to darken it up a little, use a regular graphite pencil, (no. 2 or softer like a HB) and lightly fill in the area, burnish again using the top layer of colored pencil to 'smooth' the area before again burnishing. I prefer to burnish using a rounded point on the top most color and pressing with firm pressure. There are burnishing tools available that have a metal ball or spoon-like tip that works well also. Just experience and use that which flows with you. Adding the graphite allows better shading control in order to render the work in a more realistic 3D effect should you like. Use the Prismacolor black to render really dense spaces. After burnishing, the colors probably will flush (change surface color to a slightly milk-like appearance). This is normal and is corrected when you spray a top varnish over restoring the original color quickly. But do not spray too much to create a runny surface. Practice will prevail and one gets the hang of it rather fast.