Color Processing
LOQ 18-3
How do we perceive color in the world around us?
We talk as though objects possess color: “A tomato is red.” Recall the old question, “If a tree
falls in the forest and no one hears it, does it make a sound?” We can ask the same of color: If
no one sees the tomato, is it red?
The answer is No. First, the tomato is everything but red, because it rejects (reflects) the long
wavelengths of red. Second, the tomato’s color is our mental construction. As Sir Isaac
Newton (1704) noted, “The [light] rays are not colored.” Like all aspects of vision, our
perception of color resides not in the object itself but in the theater of our brain; even while
dreaming, we usually perceive things in color.
“It is in the brain that the poppy is red, that the apple is odorous, that
the skylark sings.” — Oscar Wilde, love letter to Alfred Douglas, 1896
One of vision’s most basic and intriguing mysteries is how we see the world in color. How,
from the light energy striking the retina, does our brain construct our experience of such a
multitude of colors?
, Modern detective work on the mystery of color vision began in the nineteenth century, when
German scientist Hermann von Helmholtz built on the insights of an English physicist,
Thomas Young. They knew that any color can be created by combining the light waves of
three primary colors—red, green, and blue. So Young and von Helmholtz’s research led to a
hypothesis: The eye must therefore have three corresponding types of color receptors.
Researchers later confirmed the Young-Helmholtz trichromatic (three-color) theory, by
measuring the response of various cones to different color stimuli. The retina does indeed
have three types of color receptors, each especially sensitive to the wavelengths of red, green,
or blue. When light stimulates combinations of these cones, we see other colors. For
example, the retina has no separate receptors especially sensitive to yellow. But when red and
green wavelengths stimulate both red-sensitive and green-sensitive cones, we see yellow.
Said differently, when your eyes see red and green without blue, your brain says yellow.
In Singapore, yellow taxis—which are strikingly visible—have had 9
percent fewer accidents than blue taxis (Ho et al., 2017).
By one estimate, most of us can see differences among more than 1 million color variations
(Neitz et al., 2001). Some lucky people, mostly female, can see up to 100 million colors
thanks to a genetic condition known as tetrachromatic color vision (Jordan et al., 2010).
Asked to look at a leaf, a woman with tetrachromatic color vision said, “You might see dark
green but I’ll see violet, turquoise, blue. It’s like a mosaic of color” (Ossola, 2014). Others
less fortunate, mostly male, have the genetically sex-linked condition of color-deficient
vision. Worldwide, about 1 in 12 males and 1 in 200 females are “color blind.” Most are not
actually blind to all colors: They simply lack functioning red-or green-sensitive cones, or
sometimes both. Their vision—perhaps unknown to them, because their lifelong vision
