Yellow on your screen does not exist! Part 2: The color illusion

… and magenta is an illusion of our brain

In Part 1 of this story we described the nature of colors and the magic of rainbows. Now we move to the second part where we discuss how our eyes and brain play tricks on us.

So far, we have talked about colors as a light of precisely defined wavelength of electromagnetic wave. It is a physical, measurable property that defines a color. We can wonder then what kind of biological instrument do we have in our eyes to see colors? The answer is almost magical because it is not only about the fascinating biochemistry of photoreceptors in the eye, but it is also about the image processing happening in the brain that leads to weird illusions – colors that do not exist, but we can see them!

Our eyes have three types of color-sensitive cells called cones, positioned on the retina at the back of the eye. Each type covers a wide range of color wavelengths, but with different sensitivity for different colors. This means, for example, that two different cones might be triggered by the same color, but the strength of the signal sent to the brain will differ between them. Roughly, we can call these cone types red, green, and blue (better names are short, medium, and long wavelength cones), and their overlap in sensitivity is the key behind the color vision. We should say here that about 8% of males, and 0.4% of females reading this article have some type of distortion in color perception, generally called color blindness. But the story is the same if they have two types of cones active, except that they can differentiate a smaller range of color shades.

The opposite is a rare condition of four, instead of three, color sensitive cells. Those people (so far only proven in women) can see the world around us in many more shades of color than the rest of us. In the animal world, things can go beyond human vision. Some insects can see the UV light, which we discussed in our previous story. But the strangest color vision probably goes to the mantis shrimp. This animal has 12 types of color receptors, probably with the purpose of reducing the need for a slow process of image analysis performed in the brain. In other words, it has a more complex eye to simplify the vision system.

Color illusion: the principle behind the way how our eyes work is shown in this cartoon. If the color receptors are like buckets and light like colored rain, then a purely yellow rain fills 3 red buckets and 2 green. The brain knows only about these numbers of buckets. But then the brain cannot differentiate this signal from the one where a green-red rain fills 2 green buckets and 3 red buckets. The brain thinks we see yellow light in both situations.

The reason why we can see all the colors of rainbow with only three types of color receptors is hidden in the brain. When a rainbow color enters our eyes, it stimulates our color-sensitive cells. Each cell type is stimulated to a certain level, depending on the color wavelength. The brain receives only the information on how much stimulation was present in each of the receptors. Based on these three numbers, the visual processing part of the brain concludes that we see some color.

This process is hidden from our consciousness and conscious control. We can be aware only of the final conclusion drawn by the visual cortex of the brain. This is why our brain can be fooled by a color illusion when the very same combination of the three stimulations in color receptors is created by a completely different combination of colors.

For example, if you read this text on a digital screen of your computer, smartphone, or TV, the yellow color you see does not exist! There is no yellow light at all! What you see is green and red colors falling together on your color receptors in the eye. These two colors stimulate the red and green receptors to the same levels that a pure yellow light would do. The brain cannot figure out the difference and it erroneously concludes that this is yellow.

Digital screens under a microscope are revealed as mixtures of only three types of pixels: blue, green, and red. A white screen (A) appears when all three pixels shine brightly, while red (B) requires only red pixels.

Thanks to this brain mistake we can have screens made of only three light emitting diodes: red, green, and blue. If you look at any digital screen with a microscope, you will see how colors are broken down into the brightness levels of red (R), green (G), and blue (B) pixels. You can control the brightness of each RGB pixel to produce any color you want.

Red-green-blue control of color in PowerPoint: select “More Fill Colors” and then “Custom”. This will open a window where you can control the brightness of red, green, and blue pixels using values from 0 (pixel is off) to 255 (maximum pixel brightness) to create different colors.

You can make an interesting experiment combining the computer control of pixel brightness and watching the screen using a prism:

• Open a graphics program or a presentation program (e.g., PowerPoint), turn the background screen color to black and then make a line just a couple of pixels wide

• Open the RGB control of the line color and play with the RGB values

• Look at the line through prism. Even better, if you have a projector, put the prism in front of the line and observe the colors produced by the prism.

A prism is positioned (A) in the front of magenta line projected to the wall (B). The prism breaks this magenta light into the blue and red spectral lines (C).

This experiment demonstrates how some colors produced by the screen do not exist. For example, if you use values RGB=(255,255,0), you will see yellow, but the prism will break this into red and green line (note: yellow might appear between them if the projected line is not narrow enough). Similarly, combination RGB=(0,255,255) produces cyan.

But the most interesting one is RGB=(255,0,255), which gives magenta.  It is a mixture of blue and red and it is not a rainbow color. This means, magenta does not exist as a color. It is a complete illusion that exist only in our brain. Unlike in the case of cyan and yellow, there is no magenta light wavelength. Magenta exists only as at least two wavelengths. If you play with different values of R and B pixels, you will discover a range of colors that are an illusion, too.

Interestingly, even though purple is one those illusions and not a rainbow color, it can appear in bright rainbows because of one additional wave phenomenon called interference that mixes rainbow colors. But this deserves a whole new story when we will talk about the optical illusion of shiny colors of beetles.