Visible light is a form of electromagnetic radiation perceptible to the human eye. We see different colors depending on the wavelengths of visible light that reach us.

The visible light spectrum is the part of the electromagnetic spectrum that spans wavelengths from 380-750 nanometers (1 nanometer is one-billionth of a meter, or about the diameter of a hydrogen atom) and includes all of the colors of the rainbow – red, orange, yellow, green, blue, indigo and violet – that the human eye can perceive.

The different wavelengths of light are what define which colors we see. For example, wavelengths of approximately 680-740 nanometers look red while a wavelength of 550 nanometers is yellow-green.

[insert diagram of visible light spectrum with colors and wavelengths]

White light, like what a standard light source gives off, is a combination of all the colors together. It can be split into a rainbow using a prism.

Black, on the other hand, is actually the absence of any color – it is what the eyeball "sees" when no light reaches it from the location that appears black.

Color perception is governed by cone cells in the human eye, which can detect red light, green light and blue light. Each eyeball has 5 million to 7 million cone cells that can be stimulated in a variety of ways.

Depending on the relative intensity of light and which wavelengths reach the cells, the combination of red, green and blue reception allows a human to see all the colors in the visible light spectrum. This process is akin to how an RGB color video screen generates more than three colors for viewers.

People who are colorblind have deficits in their cone cells. Most people who are colorblind are actually only partially unable to see some colors; being unable to see any color at all is very rare. Color blindness affects about 8 percent of men and 0.5 percent of women (as a sex-linked disorder, men are more likely to be affected since they only have one copy of the X chromosome).

Artists follow a set of guidelines known as color theory to produce aesthetically pleasing designs. Color theory describes several ways to mix different colors using Isaac Newton's color wheel. The wheel divides colors into three main categories:

• Primary colors: red, green, and blue (the same as the colors perceived by cones in the eyeball)
• Secondary colors: created by mixing two primary colors
  
• Tertiary or intermediate colors: created by mixing a primary and a secondary color
  

Mixing two primary colors to make a secondary color is also known as additive color mixing. Subtractive color mixing, on the other hand, combines the basic colors cyan, magenta and yellow as overlapping layers to create black, or an absence of color.

Modern color theorists use these principles of the color wheel to create five major color schemes: analogous, complementary, split-complementary, triadic and tetradic. Each of these color systems is meant to achieve harmony in a design.

In addition to dividing colors into categories, color theory designates colors as warm (yellow or red), cool (blue, green, or purple) or neutral (brown, gray, black, or white). These groupings are thought to describe the emotional responses people have to color.

Three relative attributes define color: value, chroma and hue.

• Value, sometimes called lightness, describes the overall intensity (brightness) of a color.
• Chroma refers to the saturation of a hue, or how much one color dominates. For example, a purple could be more on either the reddish or bluish side rather than equally both.
• Hue is just another word for color or a shade.