Responding to different color angles the eye lens must change focus to move from red to green in a painting just as it would change focus to move from the barn to the mountain in nature.
In nature, the change of focus means three-dimensional depth. But on a flat painting, the same change of focus means an optical illusion of depth.
The recession of green away from red is not on the canvas, which is flat, but is an optical illusion which takes place in the eye and the brain.
Now that we understand the optical illusion as it relates to the use of two complementary colors, we no longer need confine ourselves to the three traditional steps between complementary pairs in order to induce the optical illusion that colors advance and recede. We can recede 1 step, 2 steps, 3 steps, 4 steps or we can use the entire spectrum to recede 5 steps deep into our picture.
The Red/Violet picture was designed to illustrate the optical illusion of depth that occurs when red is on the retina and violet recedes visually five steps, the deepest color separation in the spectrum. When the eye is focused on the objects in the red foreground, the violet objects in the background are in peripheral vision, 5 steps out of focus. When the violet objects are in focus on the retina, red has advanced beyond focus by 5 steps.
5 4 3 2 1 focal steps