Where is the BROWN in the rainbow?

All colors in the rainbow (called spectral colors) are associated with a particular wavelength of light. All else equal, light at about 450nm will typically appear blue, light at 605nm will typically appear orange, and light at 740nm will typically appear red.

Some colors don't have a single wavelength associated with them. These nonspectral colors (e.g., beige, puce, cornflower blue) can be created by combining multiple wavelengths together.

For example, magenta is nonspectral: we can perceive it when short wavelength and long wavelength light appear simultaneously. There is no single wavelength associated with magenta.

Given the apparent absence of brown in the rainbow, you might expect that it is nonspectral, like magenta is. Perhaps the only way to make brown is to combine multiple wavelengths.

However, brown is—in a way—right there in a rainbow! It can indeed be associated with a single wavelength of light.

So why can't you see the brown in the rainbow? The answer depends on how our eyes and minds create our experience of color.

We start with orange,
and turn down the brightness.

Watch the video below to show how changing brightness changes the circle on the right from orange to brown.

Brown = dark orange!

Another way to demonstrate the relationship between brown and orange is to take a light filter that is described as brown and shine light through it. . .

It looks orange!

This means it's impossible to have a brown flashlight: if there is enough light present for it to be visible, it will appear orange!

So the same stimulus may look brown or orange depending on how bright it is. . .or on how bright we believe it is.

In the checkerboard below, the two circles are exactly the same color (hue, saturation, and brightness).

However, we tend to interpret the one near the bottom as brighter (more orange) than the one near the top.

The one nearer the bottom appears to be in shadow, so we are correcting for that perceived darkness and seeing it as brighter (oranger) than it is.

These examples reveal just how tightly linked brown and orange are. They are only distinguished by brightness, or by our expectations of brightness.

Is it surprising that brown and orange are so closely related?

It’s not surprising that starting with red and turning down the brightness leads to . . . dark red. Even if we call it maroon instead, it isn't surprising that making red somewhat darker leads to maroon, because we generally think of maroon as a type of red. This is not the case with brown; most people would not describe brown as a type of orange. Brown and orange feel psychologically different in a way that most shades that share hue and saturation don't.

This may be partially driven by our language.

The English word for brown comes from Old English and has been used as a color term since 1000CE. In contrast, the word "orange" didn't exist in Old English. The closest thing was "geoluhread” (yellow-red). In 1390, Chaucer described a fox as having a color “color was betwixe yelow and reed" because there didn't yet exist in English a word specifically for orange. But by the mid 1600s, the word was commonly used in English.

What happened between 1390 and 1600?