You can apply it by filling it tightly to give it a density and intensity of shade.
You can gently apply pigment to get light overflows
When you apply the pigment on a dark base you can combine contrasting shades and place them next to each other but only if the shades are manifested not in the intensity of the color, but with a slight shading. As in this case the dark base absorbs all the "dirty" combinations. This way you can create the effect of "gasoline" spots and combinations.