How does the color wheel work?

[Meatball]

According the color wheel, red and green are "complimentary" because they are on opposite sides of the color wheel. Complimentary colors are colors that look good against each other. If one wants to dull one color, he may add its compliment to the original color.

Now... who the heck came up with the color wheel? Who decided to put red next to purple? They are after all on opposite sides of the visible spectrum...
Why is it that colors on opposite sides of the wheel compliment each other? Is there some kind of math behind this? Are some red wavelengths multiples of some green wavelengths or something? Is there some kind of pattern that can be used to determine the "compliment" of 564nm? How does this all work?

 

[dr-ebert]

There have been many different color wheels, color balls, and color planes in use throughout history. So in the end it's just a matter of convention.

For "complementary" colors it's a bit different. First of all, I guess you know that the eye has 3 types of receptors: for red, green, and blue light. Light of a certain wavelength stimulates all 3 of them, some more, some less; and this signal combination is interpreted by the brain as a certain color. That's why TV screens and computer displays have red, green and blue pixels: by making them glow with a certain brightness, you can create all colors. This RGB scheme is used in "additive" color mixing: you add one or more of the 3 wavelengths to simulate any other.

In printing, you use "subtractive" color mixing, since dyes/inks just take wavelengths away from the ambient white light. Here you use cyan/magenta/yellow (look at a printed color picture) which are the complementary colors to red/green/blue. The rule-of-thumb formulas (with W for white):

W = R + G + B

C = W - R = G + B
M = W - G = R + B
Y = W - B = R + G

R = W - G - B = M + Y
G = W - R - B = C + Y
B = W - R - G = C + M

 

[RA_pierce]

When you're talking about dyes, ink, and pigments, you use subtractive color mixing.
For example: Red and Green are opposite on the color wheel. When you combine them in equal amounts, they produce a brownish grey. Same with yellow and purple, blue and orange, and so on.
The primary colors of pigments are Red, Yellow, and Blue (in printing it's CMYK which stands for cyan, magenta, yellow, and black). With those colors combined in varying proportions, you can create just about any color you wish. But combining them all in equal amounts will give you a dull, dark grey.

When you're talking about light, color mixing is additive, and the color wheel is different.
Although red and green are complimentary, combining the two wavelengths will give you yellow. Blue and green makes cyan, red and blue makes magenta, etc.
The primary colors of light are Red, Green, and Blue. Combining these in the correct amount will give you white (sunlight is white, and consists of the full visible spectrum).

Another difference is that when painting you can add black to a color to make it darker. There is no black wavelength (black is rather the absence of light) so you cannot make a color "darker" -only dimmer by reducing power.

Colors that cannot be recreated using light (RGB) are grey, brown, and black. The browns you see on LCD screens are dim orange/reds and the greys are dim whites.

 

[nikokapo]

Great answers, guys. Thanks for creating this thread, I had some serious doubts about this too !

 

[Crossfire]

Who decided to put red next to purple?

Purple != Violet

Violet is a true spectral color, usually from 400nm to 430nm wavelengths. Purple is a combination of red and blue and is not a spectral color.

Most people use the two interchangeably.