In this example the background has been hatched with alcohol pens. This requires only three different colors. Colors blend nicely on paper and new shades will form.
This technique was originated by accident, when I planned to color the background in turquoise, but the ink was just about to end.
Then I just drew lines as long as that ink lasted, and the rest of the background I hatched with other colors.
This style is especially suitable for a big, blank background, and the result is very impressive. You can create different kind of moods and depth perceptions, depending on the direction of the lines.
Pens: Touchfive markers
Book: Mental Images vol 1