Abstract: When complementary-gelled lights are falling on the same plane, they can easily rob each other of color. So it is important to make sure your lights are hitting different areas, with minimal overlap.
Above is a two-speedlight portrait against a white wall. White walls are the natural enemy of a gel, and practically live to wash out your color. Especially when using two flashes with dense, complementary gels. Knowing how to keep your multi-colored lights operating on different planes will help you retain more saturated color.