The idea is to fill an area by coloring the fabric behind the embroidery. Usually, this means that only the outline is embroidered, with some details added. However, tinting isn't only for coloring inside the lines. Many artists use this technique to create spectacular backgrounds, borders, and accents for their embroidery work.
A Variety of Pressure Creates Emphasis. If there are some areas you want to accent, you can press a little harder on the crayon and end up with a darker, more intense color. This is another good reason to color lightly for full coverage. For example, in this picture of the heart below, I pressed harder when tracing the lines within the heart and in the vein and artery openings. I did the same thing when coloring the muscles and the brain.