The Visual System: Learn It 3—Color Vision

Color Vision

Color adds depth and richness to how we experience the world. But how does the visual system interpret color from light waves?

Normal-sighted individuals have three types of cone cells in the retina, each most sensitive to a different wavelength of light. Together, these cones allow us to perceive millions of color variations.

trichromatic theory of color vision

According to the Young-Helmholtz trichromatic theory of color vision, shown in Figure 1, all colors in the spectrum can be produced by combining red, green, and blue.

Each cone type responds maximally to one of these three wavelengths:

  • Short wavelengths (blue)

  • Medium wavelengths (green)

  • Long wavelengths (red)

When different cones are activated together, the brain blends their input to produce the full range of perceived colors.

 

A graph is shown with “sensitivity” plotted on the y-axis and “Wavelength” in nanometers plotted along the x-axis with measurements of 400, 500, 600, and 700. Three lines in different colors move from the base to the peak of the y axis, and back to the base. The blue line begins at 400 nm and hits its peak of sensitivity around 455 nanometers, before the sensitivity drops off at roughly the same rate at which it increased, returning to the lowest sensitivity around 530 nm . The green line begins at 400 nm and reaches its peak of sensitivity around 535 nanometers. Its sensitivity then decreases at roughly the same rate at which it increased, returning to the lowest sensitivity around 650 nm. The red line follows the same pattern as the first two, beginning at 400 nm, increasing and decreasing at the same rate, and it hits its height of sensitivity around 580 nanometers. Below this graph is a horizontal bar showing the colors of the visible spectrum.
Figure 1. This figure illustrates the different sensitivities for the three cone types found in a normal-sighted individual. (credit: modification of work by Vanessa Ezekowitz)

A Real-Life Example: Discovering Colorblindness

William, a single father, was preparing for a public event when his 7-year-old daughter told him his clothes didn’t match. Concerned, they sought a second opinion from a nearby convenience store. The store clerk examined William’s attire—a bright green pair of pants, a reddish-orange shirt, and a brown tie—and confirmed, “Your clothes definitely don’t match.”

Prompted by these comments, William consulted friends and coworkers, who diplomatically described his style as “unique.” Realizing something might be off, he visited an eye doctor and discovered he was colorblind, unable to distinguish between certain shades of greens, browns, and reds.

The figure includes three large circles that are made up of smaller circles of varying shades and sizes. Inside each large circle is a number that is made visible only by its different color. The first circle has an orange number 12 in a background of green. The second color has a green number 74 in a background of orange. The third circle has a red and brown number 42 in a background of black and gray.
Figure 2. The Ishihara test evaluates color perception by assessing whether individuals can discern numbers that appear in a circle of dots of varying colors and sizes.

 

Although William’s condition rarely affects his daily life, his story shows how sensory limitations can go unnoticed until a specific situation reveals them.

Color Vision Deficiencies

Total color blindness—seeing only shades of gray—is extremely rare and results from having only rods (no cones), which causes low visual acuity and poor daylight vision.

The most common color vision deficiency is red–green color blindness, an X-linked inherited trait (Birch, 2012).

  • About 8% of European males, 5% of Asian males, and 4% of African males have red–green color deficiency.
  • It is less common in Indigenous American, Australian, and Polynesian males (under 2%).
  • Only about 0.4% of European females experience this condition, since females would need to inherit the gene from both parents.

Opponent-Process Theory

The trichromatic theory of color vision is not the only theory—another major theory of color vision is known as the opponent-process theory.

opponent-process theory

According to opponent-process theory, color is coded in opponent pairs: black-white, yellow-blue, and green-red. The basic idea is that some cells of the visual system are excited by one of the opponent colors and inhibited by the other. So, a cell that was excited by wavelengths associated with green would be inhibited by wavelengths associated with red, and vice versa.

This pairing mechanism also explains negative afterimages—the lingering visual impression that appears after looking away from a bright or colored image.

If you stare at a red shape for several seconds, your red-sensitive cells become fatigued. When you then look at a white background, the opponent color (green) appears instead. Try it with the flag below!

 
An illustration shows a green flag with a thick, black-bordered yellow lines meeting slightly to the left of the center. A small white dot sits within the yellow space in the exact center of the flag.
Figure 3. Stare at the white dot for 30–60 seconds and then move your eyes to a blank piece of white paper. What do you see? This is known as a negative afterimage, and it provides empirical support for the opponent-process theory of color vision.

Integrating the Theories

These two theories aren’t contradictory—they describe different stages of visual processing:

  • Trichromatic theory explains color detection at the retina, where cones respond to three wavelengths (red, green, blue).
  • Opponent-process theory explains how color signals are interpreted beyond the retina, as information travels through the optic nerve and into the brain.

Together, they provide a complete picture of how the human visual system perceives color—from the first detection of light waves to the rich, dynamic color experience we enjoy.