Are black, white, and gray colors?

Short answer

Physically, black, white, and gray are surfaces that reflect all wavelengths in roughly equal proportion; perceptually, they are colors with no hue, only lightness, which is why they're called "achromatic" — colorless colors. A completely neutral gray, a surface that reflects every visible wavelength in exactly equal amounts, doesn't exist in real life; laboratory standards come very close, but every gray in paint, fabric, and concrete leans slightly warm or cool. What's more, even a flawless gray stops looking gray under colored light or in a colored surrounding.

Two definitions: physics and perception

The question "Is black a color?" bounces between two different definitions.

In physics, a surface's color is determined by the proportions in which it reflects visible wavelengths; BI-04 covered these reflectance curves. A red tomato reflects a lot of long wavelengths and little of the short ones; its curve tilts in one direction. The curves of white, gray, and black surfaces, by contrast, are flat: all of them reflect every wavelength in equal proportion. The only difference between them is the curve's height; white reflects most of it, gray reflects a portion, black reflects very little.

In perception, the standard is different. A color is described by its hue, saturation, and value — DT-01's three dimensions. Black, white, and gray are colors with no hue and no saturation, only value. In the CIELAB space from BI-19, these are the colors that sit at zero on both color axes and line up only along the lightness axis.

The two definitions mostly overlap, but where they diverge is instructive. In everyday language, Turkish uses the word "color" for things that have a hue, and people say "black-and-white photo"; color science, however, counts achromatic colors as colors too. Both are correct in their own context. Telling a house painter "black is a color" makes sense, and so does telling a physicist "black is the absence of light"; but a black surface never absorbs light completely — it only reflects very little of it.

Gray, relative to its surroundings

The lightness of achromatic colors isn't absolute. In BI-14, we saw that the same gray squares look different in lightness depending on their surroundings. This effect reaches its most extreme form in achromatic colors.

In a classic demonstration attributed to the Gestalt psychologist Adhémar Gelb, a black sheet of paper is shown in a dim room, lit only by a spotlight aimed at it. Observers see the paper as white. The moment a real white sheet of paper is held in front of the black paper, inside the spotlight's beam, the black paper suddenly turns black; when the white paper is pulled away, it looks white again. The light falling on the paper never changes; what changes is the presence of a white reference to compare it against.

Black itself is a relative perception too. What we see when we open our eyes in pitch darkness isn't pure black but a dark gray. This tone, which the German physiologist Ewald Hering called "Eigengrau," or "inner gray," comes from the spontaneous signals that cells in the retina produce even without light. A starlit night sky looks darker than this gray, because the bright points of the stars give it a contrast to be seen against. The deepest blacks are seen when there's something bright next to them.

Is there such a thing as a completely neutral gray?

The question on the lesson card depends on how strict the physical definition is. A completely neutral gray would be a surface that reflects every visible wavelength in exactly the same proportion. Such a surface exists as an ideal concept; it doesn't exist in real materials, but some come surprisingly close to it.

The reference surfaces used in measurement laboratories are the best example of this. White reflectance standards, made from a type of PTFE powder that is compressed and sintered, reflect more than 99 percent of light across most of the visible spectrum, with deviations in that region below five in every thousand. The same material is mixed with black pigment to produce spectrally flat gray standards as well. These surfaces are used as the "zero point" in calibrating color measurement devices.

Everyday grays, though, aren't like this. A wall paint, a wool fabric, a concrete surface, or a steel sheet carries a slight tilt in its reflectance curve. If the curve rises a little toward long wavelengths, the gray shifts warm, toward brownish or beige; if it rises toward short wavelengths, it shifts cool, toward bluish or steel gray. That's why paint catalogs contain hundreds of different "grays," and designers choose "warm gray" and "cool gray" as separate colors.

There's also the matter of light. Even the flawless gray in the lab reflects orangish light under an incandescent lamp, and bluish light in the shade, under skylight. The eye mostly doesn't notice this shift; that a camera needs a setting to correct for it was the subject of FS-01. So "neutral" is a combination of the surface, the light, and the eye's state of adaptation all at once. The honest answer to the question on the lesson card is this: a neutral gray exists within a measurable margin of error; in an absolute sense, it does not.

Surprising blacks

Even black has its own blacks. A large black area printed with only black ink — that is, a hundred percent K — often looks paler and slightly grayish than expected; the ink doesn't fully cover the paper, and light reflecting through the gaps lightens the black. That's why printers add specific proportions of cyan, magenta, and yellow ink underneath the black for large black areas; this is called rich black. By varying the proportion of the added colors, the black can be pulled slightly cool or warm.

Blacks on screen differ too. The black of a backlit liquid crystal display looks dark gray in a dark room, because it can't fully block the light; displays where every pixel produces its own light, on the other hand, can switch the light off completely for a black pixel. In the lab, coatings made from carbon nanotubes have trapped light almost completely and joined the ranks of the darkest surfaces ever made by human hands; the story of these blacks, and the controversy they caused in the art world, is the subject of MA-06.

Example — the words "black" and "white" used for people

In everyday language, the words "black" and "white" are used for people. These words aren't a color measurement; they're historical and social categories. From the standpoint of color science, no human skin tone is achromatic.

In BI-19, we had calculated four example skin colors in CIELAB. The lightest of these samples had a lightness of about L 83, and the darkest about L 21; but both had a noticeable chroma, about 20 in the darkest sample and about 23 in the lightest. For comparison, a neutral gray has a chroma of zero. So the lightest skin isn't white, and the darkest skin isn't black; both, like every skin tone in between, are colors with hue and saturation.

This distinction has a practical consequence too. When a photographer or designer shows a dark-skinned face against a black background, or a light-skinned face against a white background, the goal should be to preserve the skin's own hue and saturation. A black background doesn't "swallow" the face, but if exposure or color correction pulls the skin toward gray, the skin's warmth and chroma disappear. Skin should remain a color even in front of an achromatic background.

Common misconception

Summary

1. In physics, black, white, and gray are surfaces that reflect every wavelength in equal proportion; in perception, they are achromatic colors with no hue and no saturation, only lightness.
2. The lightness of achromatic colors is relative: in the Gelb effect, black paper under a spotlight looks white, while in pitch darkness what's seen is not pure black but a dark gray.
3. A completely neutral gray exists only within a measurable margin of error; everyday grays lean warm or cool, and blacks, too, are produced at different depths in print and on screen. Human skin tones, from the lightest to the darkest, are not achromatic.

Test yourself

1. (Easy) What is an achromatic color?
2. (Easy) How is rich black achieved in printing?
3. (Medium) Does a completely neutral gray exist in real life?
4. (Medium) What does the Gelb effect show about the perception of lightness?
5. (Hard) Why do we see a dark gray rather than pure black in pitch darkness, and why does a starlit sky look darker than this gray?
6. (Hard) Why don't the words "black" and "white" used for people describe achromatic colors, from the standpoint of color science?

Answers

1. A color with no hue and no saturation, defined only by its lightness; white, the grays, and black ("Two definitions: physics and perception").
2. By adding specific proportions of cyan, magenta, and yellow ink underneath the black ink; this gives a deeper black in large areas ("Surprising blacks").
3. Not in an absolute sense: no material is known that reflects every wavelength in exactly equal proportion. Laboratory standards come very close to it, with deviations smaller than five in every thousand in the visible region; everyday grays, meanwhile, lean slightly warm or cool, and every gray looks different depending on the light and its surroundings ("Is there such a thing as a completely neutral gray?").
4. That lightness is determined not by the absolute amount of light but by comparison with the brightest surface in the scene; black paper under a spotlight looks white until a real white is placed next to it ("Gray, relative to its surroundings").
5. Cells in the retina produce spontaneous signals even without light, and these signals are perceived as a dark gray; in a starlit sky, the bright points create a contrast, which is why the sky looks darker ("Gray, relative to its surroundings").
6. These words are historical and social categories, not a color measurement. Even the lightest and darkest skin samples have a noticeable chroma in CIELAB; a neutral gray's chroma, by contrast, is zero ("Example — the words \"black\" and \"white\" used for people").

Sources

• Labsphere, "Spectralon Diffuse Reflectance Standards," product and technical documentation.
• Gilchrist, A., Seeing Black and White, Oxford University Press, 2006.
• Hering, E., Grundzüge der Lehre vom Lichtsinn, 1920.

Next step

DT-07 blends this lesson's achromatic colors with chromatic ones: it covers the tints, shades, and tones you get by adding white, black, or gray to a color, their Turkish names, and why adding black shifts a color's hue. MA-06, meanwhile, goes to this lesson's most extreme black: the coatings that trap light in forests of carbon nanotubes, the exclusive-license controversy, artists' reactions to it, and how these blacks are used in space and in optics.