What is Color?
A physical and perceptual introduction — from the wavelengths of light to the receptors in the eye that turn them into the experience of color.
Light — the physical side
Light is a form of electromagnetic radiation. What we call "visible light" is a narrow band of wavelengths that human eyes can detect — roughly 380 to 700 nanometres. Within that band, shorter wavelengths appear violet, longer wavelengths appear red, and the middle contains every other hue.
| Wavelength | Perceived color |
|---|---|
| 380–450 nm | Violet |
| 450–495 nm | Blue |
| 495–570 nm | Green |
| 570–590 nm | Yellow |
| 590–620 nm | Orange |
| 620–750 nm | Red |
White light — such as sunlight — contains all visible wavelengths together. When white light strikes an object, some wavelengths are absorbed by the object's surface and others are reflected. The reflected wavelengths reach the eye, and the brain interprets them as the object's color. A red apple absorbs blue and green light and reflects red.
The eye — the perceptual side
Inside the human retina there are two types of photoreceptor cells: rods and cones. Rods are sensitive to low light and contribute only to brightness. Cones are responsible for color vision, and they come in three types, each sensitive to a different range of wavelengths:
- S-cones — most sensitive to short wavelengths (blue region).
- M-cones — most sensitive to medium wavelengths (green region).
- L-cones — most sensitive to long wavelengths (red region).
Any incoming light stimulates all three cone types to different degrees. The brain compares the three signals and produces the experience of a specific color. This is why the RGB model works so well on screens — it directly maps to the three cone types.
Is color "real"?
This is one of the most discussed questions in color science. The physical reality is that light has wavelengths and objects reflect different wavelengths — that part is objectively real. But the experience of color — the subjective "redness" of red — is a product of the brain's processing. It exists in the perceiver, not in the light.
This becomes especially clear when you consider color vision deficiency. Two people can look at the same object under the same light and see different colors, because their cone sensitivities differ. The light is identical; the perception is not. Color is best understood as a relationship between light, surface and perceiver.
Additive vs subtractive
There are two fundamentally different ways colors can be produced:
| Type | How it works | Examples |
|---|---|---|
| Additive | Colors are made by emitting light. Adding more light makes things brighter. | Screens, projectors, LEDs — using RGB |
| Subtractive | Colors are made by absorbing (subtracting) light. Adding more pigment makes things darker. | Printing, paint, ink — using CMY(K) |
Because screens emit light and paint absorbs it, they need different primaries. This is why "what are the primary colors" has more than one correct answer — it depends on whether you are talking about light or pigment.
Why it matters for design
- Colors are device-dependent. A color that looks identical to you may appear slightly different on another screen. This is why wide-gamut spaces exist.
- Not everyone sees color the same way. Approximately 8% of men have some form of red-green color vision deficiency. Always pair color with other signals.
- Lighting affects perception. The same color looks different under tungsten, daylight or LED light. Screen brightness and color temperature matter.
- Context changes everything. The same gray square appears darker against white and lighter against black. Adjacent colors affect perception.
Continue reading
From here, the next topics in the encyclopedia explain how color is encoded numerically:
Frequently asked questions
What is color?
Color is the result of the human visual system interpreting different wavelengths of light. Physically, light is an electromagnetic wave with a wavelength; perceptually, the brain turns those wavelengths into the experience of color.
Are colors real?
It depends on what you mean. Wavelengths of light are real physical phenomena. The subjective experience of color — the "redness" of red — is a product of the brain's processing. Color exists in the interaction between light and the perceiver.
What is the visible spectrum?
The range of electromagnetic wavelengths the human eye can detect, roughly 380 to 700 nanometres. It spans from violet (shortest) through blue, green, yellow, orange, to red (longest).
How do screens create color?
Screens emit light. Each pixel contains tiny red, green and blue subpixels whose brightness can be controlled individually. By varying the amounts of red, green and blue light, the screen creates the appearance of almost any color. This is additive color mixing.
Do animals see the same colors we do?
No. Many animals have different cone configurations. Dogs have two cone types (dichromatic vision) and see a reduced range. Birds and some insects have four or more cones, allowing them to see ultraviolet and other colors invisible to humans. Mantis shrimp have twelve or more photoreceptor types.