Color is the perceptual quality of light. (Color is a subjective response by the brain to light stimulating the retina.)
Two visual regions have the same color if a difference between them cannot be perceived by the average human eye.
The human eye can distinguish nearly ten million colors.
Color as a visual response should not be confused with the "color" of a pigment, which is the color one would see when viewing that pigment under typical lighting conditions.
Although they may not satisfy the definition of a color in a fashion sense; black, white, and gray each satisfy the physical and perceptual definitions of a color.
The color of the light coming from an object has its origin in one or more of the following processes…
emission: the object itself is a source of light with a color determined by its spectra
reflection: certain frequencies are reflected from the object while others are not
transmission: certain frequencies are transmitted through the object while others are not
interference: certain frequencies are amplified by constructive interference while others are attenuated by destructive interference
dispersion: the angular separation of a polychromatic light wave by frequency during refraction
scattering: the preferential re-radiation of certain frequencies of light striking small, dispersed particles
There are six simple, named colors in English (and many other languages) each associated with a band of monochromatic light. In order of increasing frequency they are red, orange, yellow, green, blue, and violet.
The range of frequencies corresponding to each band is subject to individual, cultural, and historical factors.
Indigo is not included in this list as it is purely a historic artifact. The word is rarely used by contemporary speakers of English to describe a color.
The sensation of purple cannot be produced using light of a single frequency, but only by combining light near the red end of the spectrum (the lower frequencies) and the violet end of the spectrum (the higher frequencies).
At relatively low intensities…
monochromatic light in the red, orange, and yellow bands appear brown.
monochromatic light in the blue band is difficult to distinguish from violet.
Humans perceive polychromatic mixtures of light as a single color,
which may or may not look like light from a monochromatic light source.
Polychromatic light is described physically by a spectral power distribution (often just called a spectrum), which is a graph of spectral power density vs. wavelength (or frequency)
Regions with different spectra that appear to be the same color are called metamers and the effect is called metamerism.
Regions that are metamers when illuminated by one light source may not appear to be the same color when illuminated by another light source.
The wide variety of colors visible to humans can be approximated by mixing only a small subset of colored light sources or colored pigments.
White light is a mixture of visible frequencies of electromagnetic radiation whose appearance approximates that of a blackbody radiator with its peak wavelength in the middle of the visible spectrum.
There is no one frequency distribution that can be identified as white light. Human vision adapts to the illumination provided by the environment so that many blackbody and non-blackbody sources appear white.
The quality of white light emitted from a blackbody radiator is a function its temperature. This quality is known as color temperature.
Daylight at midday is often considered the standard value of white light. It produces the same response in the human eye as a blackbody radiator at 6500 K. Visual regions with the same color temperature as midday light often appear neutral white.
A visual region with a color temperature below 6500 K emits white light that looks reddish in comparison. For cultural reasons light of this color is called warm white even though it is from a "colder" source.
A visual region with a color temperature above 6500 K emits white light that looks bluish in comparison. For cultural reasons light of this color is called cool white even though it is from a "hotter" source.
A visual region looks gray if the light from it is similar to white light, but has an overall power density somewhat lower than its surroundings.
Black is the relative absence of visible light.
A visual region that emits, reflects, or transmits much less visible light than its surroundings looks black.
The primary colors of the human visual
system are red, green,
and blue.
Primary colors
black
+
red
=
red
black
+
green
=
green
black
+
blue
=
blue
No combination of two primary colors can reproduce a third primary color.
Combinations of the primary colors will reproduce a wider range of colors than than can be reproduced using any other three colors.
Combinations of primary colors follow the rules of additive color mixing.
Additive color mixing rules
no light
=
black
red
+
green
=
yellow
green
+
blue
=
cyan
blue
+
red
=
magenta
red + green
+
blue
=
white
Systems that work by additive color mixing include…
photographic and movie film (prints, slides, negatives)
television and computer displays (CRT, LED, LCD, plasma)
The secondary colors of the human visual system are cyan, magenta, and yellow.
A secondary color is formed by subtracting a primary color from white light.
Secondary colors
white
−
red
=
cyan
white
−
green
=
magenta
white
−
blue
=
yellow
Every secondary color is the complementary color or opposite color of a primary color.
Combining primary and secondary colors of light produces light that looks white.
Complementary color mixing rules
red
+
cyan
=
white
green
+
magenta
=
white
blue
+
yellow
=
white
Combinations of secondary color pigments follow the rules of subtractive color mixing.
Subtractive color mixing rules
no pigment
=
white
cyan
+
magenta
=
blue
magenta
+
yellow
=
red
yellow
+
cyan
=
green
cyan + magenta
+
yellow
=
black
The black produced by mixing the three secondary colors is of low quality.
Systems that work by subtractive color mixing include…
three-color printing
pigment mixing (as in custom paints)
The "primary colors" of the painter's color wheel are red, yellow, and blue
When combining pigments in equal quantities…
"Painter's" color mixing rules
no pigment
=
white
red
+
yellow
=
orange
yellow
+
blue
=
green
blue
+
red
=
purple
red + yellow
+
blue
=
brown
Purple is similar to magenta.
The misidentification of these colors as "primary" is a historical artifact. A greater range of colors can be reproduced using cyan, magenta, and yellow than can be reproduced using red, yellow, and blue.
Although this is called the painter's color wheel, no serious painter would claim it possible to reproduce every desired color from these three pigments.
Color Spaces
All color spaces have at least three dimensions
RGB (red, green, blue)
Named for the dominant wavelength of the three light sources used.
Numbers ranging from zero to some bit number maximum (255, 65535, etc.) are used to describe the relative intensity of each of the three light sources.
black: none of the light sources are turned on (R = G = B = 0)
white: all light sources are turned up as bright as they can (R = G = B = maximum value)
CMY (cyan, magenta, yellow)
Named for the secondary color of the three inks when viewed under white light on a white sheet of paper.
Numbers ranging from 0% to 100% are used to describe the per cent coverage of a blank sheet of white paper by each of the three inks.
white: no ink on a white sheet of paper (C = M = Y = 0%)
black: paper completely covered with each type of ink (C = M = Y = 100%)
