Color is a function of the human visual system, and is not an intrinsic property. Objects don't "have" color, they give off light that "appears" to be a color. Spectral power distributions exist in the physical world, but color exists only in the mind of the beholder.
Start with monochromatic light — that is, light of a single frequency. The visible spectrum ranges from roughly 700 to 400 nm. If I shine light of a single frequency at your eye and dial the wavelength from 700 nm to 400 nm this is roughly what you'd see.
How many colors are there in this swatch? How many were you taught in elementary school?
The simple named colors are mostly monosyllabic in English — red, green, blue, brown, black, white, gray. (Yellow is the one exception to this rule, but it's still pretty simple.) Brevity indicates a pre-English, Anglo-Saxon origin. Monosyllabic words are generally the oldest words in the English language — head, eye, nose, foot, cat, dog, cow, eat, drink, man, wife, house, sleep, rain, snow, sword, sheath, God, and the "four letter words" — words that go back a thousand years. Some of the names for colors are loan words from French — orange and beige, since ʒ (the "zh" sound) doesn't exist in pure English (garage is a very french word), and violet and purple, since they just sound too fancy to be anglo-saxon.
|red||réad||~870||on ðæs sacerdes hrægle scoldon hangigan bellan & ongemang ðæm bellum reade apla.
on the preist's robe should hang bells and among the bells, red apples.
King Alfred's West-Saxon version of Pope Gregory's Pastoral Care
|orange||n/a||1552||no person or persons shall put to sale by retail within this realm, any cloth or clothesof any other color or colors than is hereafter expressed; that is to say, scarlet, red, crimson, murry, violet, puke, brown-blue, blacks, greens, yellows, blues, orange-tauny, russet, marble-gray, sad new color, azure, watchet, sheeps-colour, lion-colour, motly or iron-gray.
Great Britain Statutes at Large
|yellow||geolu||~900||Uyrmas mec ni auefun uyrdi cræftum, ða ði goelu godueb geatum frætuath.
Worms did not weave me with the skills of the fates, those that the yellow cloth garment decorated.
The Leiden Riddle
siððan adam stop on grene græs, gaste geweorðad.
since Adam stepped on green grass, possessed of life.
The Genesis A, B story from the Cædmon Manuscript
That raises an interesting point. Did the English (or the Angles and the Saxons) "see" orange before the French told them about it? Did the French see orange before the Spanish told them about it? Did the Spanish see orange before the Arabs told them about it? Why does Islam identify with green? Why do Russians identify with red? Why do the Dutch groove on orange? (These are rhetorical questions. Please don't email me your answers.) Where do I put black, white, gray, purple, and brown? What the hell is indigo?
Enough about language, this is a physics book. Here's the point. There is no physical significance in these colors. It's all a matter of culture and culture depends on where you live, what language you speak, and what century it is. There is nothing special about these colors. We humans who speak English and live at the dawn of the Twenty First Century have identified the following six frequency bands (well, wavelength bands actually, since wavelength is easier to measure than frequency) in the electromagnetic spectrum as being significant enough to warrant designation with a special name. They are: red, orange, yellow, green, blue, and violet.
Where one monochromatic color ends and another begins is a matter of debate as you will see in the table below.
But wait, it gets worse. How many of you reading this learned about "Roy G. Biv" (Americans, I presume) or that "Richard of York Gave Battle In Vain" (Britons, I presume)? Who among you leaned that between blue and violet there was this special color called indigo?
Oooo, indigo. Yeah, there's a word I use a lot in everyday conversation. The only time I ever hear it is when students recite the visible spectrum. Let me state that anyone who says indigo is a color equal in importance to blue or green is a thoughtless idiot. Indigo is a color of relatively little importance. If indigo counts as a color then so should canary, and mauve, and puce, and brick, and teal, and … well, you get the idea.
How many colors are there in this swatch? How many were you taught in elementary school?
If you believe that indigo is an important color, then here's a set of spectral tables for you.
Did Richard of York give battle in vain so that future citizens in the dismantled British Empire would forever remember indigo? Did Mr. and Mrs. Biv conceive little Roy G. so that future generations of Americans might learn the true nature of light? Where the hell did indigo come from?
When Newton attempted to reckon up the rays of light decomposed by the prism and ventured to assign the famous number seven, he was apparently influenced by some lurking disposition towards mysticism, If any unprejudiced person will fairly repeat the experiment, he must soon be convinced that the various coloured spaces which paint the spectrum slide into each other by indefinite shadings: he may name four or five principal colors, but the subordinate spaces are evidently so multiplied as to be incapable of enumeration. The same illustrious mathematician, we can hardly doubt, was betrayed by a passion for analogy, when he imagined that the primary colours are distributed over the spectrum after the proportion of the diatonic scale of music, since those intermediate spaces have really no precise defined limits.
John Leslie, 1838
The human eye can distinguish something on the order of 7 to 10 million colors — that's a number greater than the number of words in the English language (the largest language on earth).
The retina …
The rods, which far outnumber the cones, respond to wavelengths in the middle portion of the spectrum of light. If you had only rods in your retina, you would see in black and white. The cones in our eyes provide us with our color vision. There are three types of cone, identified by a capital letter, each of which responds primarily to a region of the visible spectrum: L to red, M to green, and S to blue.
The peak sensitivities are 580 nm for red (L), 540 nm for green (M), and 440 nm for blue (S). Red and green cones respond to nearly all visible wavelengths, while blue cones are insensitive to wavelengths longer than 550 nm. The total response of all three cones together peaks at 560 nm — somewhere between yellow and green in the spectrum.
While red, green, and blue are spaced somewhat equally across the visible spectrum, the specific sensitivities of the L, M, and S cones are not. This might seem a little confusing, especially since the L cones aren't even closely centered on the red area of the spectrum. Fortunately, the spectral sensitivity of the cones is only one part of how the brain decodes color information. Additional processing takes these sensitivities into account
Commission Internationale de l'Eclairage
The relative response of the red and green cones to different colors of light are plotted on the horizontal and vertical axes, respectively. Values on the tongue shaped perimeter are for light of a single wavelength (in nanometers). Values within the curve are for light of mixed frequency. The point in the center labeled D65 corresponds to light from a blackbody radiator at 6500 K — the effective temperature of daylight at midday, a generally accepted standard value of white light.
|radiant energy source|
|2.73||cosmic background radiation|
|500||household oven at its hottest|
|660||minimum temperature for incandescence|
|770||dull red heat|
|1400||glowing coals, electric stove, electric toaster|
|2800||incandescent light bulb, 75 W|
|2900||incandescent light bulb, 100 W|
|3000||incandescent light bulb, 200 W|
|3100||sunrise or sunset (effective)|
|3200||professional studio lights|
|3600||one hour after sunrise or one hour before sunset (effective)|
|4000||two hours after sunrise or two hours before sunset (effective)|
|5500||direct midday sunlight|
|7000||overcast sky (effective)|
the absence of light is darkness, add light to it
the basic rules of additive color mixing.
red + green = yellow
green + blue = cyan
blue + red = magenta
|red + green + blue||=||white|
the color wheel
The additive color wheel
Three color LED display
Subtractive or complimentary colors
The basic rules of subtractive color mixing
|cyan + magenta + yellow||=||black|
A four color press: yellow, magenta, cyan, black
the color wheel
The subtractive color wheel
The painter's color wheel is a historical artifact that refuses to die. The primary colors are not red, yellow, and blue. Painters and art teachers promote this scheme. It is a convenient way to understand how to mimic one color by mixing red, yellow, and blue. But these colors do not satisfy the definition of primary colors in that they can't reproduce the widest variety of colors when combined. Cyan, magenta, and yellow have a greater chromatic range as evidenced by their ability to produce a reasonable black. No combination of red, yellow, and blue pigments will approach black as closely as do cyan, magenta, and yellow.
Johann Wolfgang von Goethe (17949-1832), student of the arts, theatrical director, and author (Iphigenia at Taurus, Egmont, Faust). Lots of interesting descriptive information on the subjective nature of color, which many physicists of his day ignored, but does not propose a physical model of color.
The theory of colors, in particular, has suffered much, and its progress has been incalculably retarded by having been mixed up with optics generally, a science which cannot dispense with mathematics; whereas the theory of colors, in strictness, may be investigated quite independently of optics.
Colour is a law of nature in relation with the sense of sight … [It] is an elementary phenomenon in nature adapted to the sense of vision …
It is not light, in an abstract sense, but a luminous image that we have to consider.
Yellow, blue, and red, may be assumed as pure elementary colors, already existing; from these, violet, orange, and green, are the simplest combined results.
That all the colours mixed together produce white, is an absurdity which people have credulously been accustomed to repeat for a century, in opposition to the evidence of their senses.
The painter's color wheel.
Color Mixing Rules from Theory of Colors [Zur Farbenlehre] (1810) by Johann Wolfgang von Goethe (17949-1832) Germany.
|bit depth||number of colors||a.k.a.|
|8 bit color||208 =||256|
|16 bit color||216 =||65,536||"thousands of colors"|
|24 bit color||224 =||16,777,216||"millions of colors"|
|32 bit color||232 =||4,294,967,296||"billions of colors"|