The Perception of Color Where does color come from? Well, you need three things in order for color to exist: an object, an observer (you!), and light.

Color originates in light. Sunlight — or white light — as we perceive it is colorless. But Isaac Newton proved us wrong with a simple glass prism. He was able to break white light into a visible spectrum of colors, from red to violet, and rejoin those colors to create white (or colorless) light. In fact, a rainbow is testimony to the fact that all the colors of the spectrum are present in white light.

As the illustration shows, when light hits a red object, that object absorbs all spectrum colors in white light except for red. The object reflects the red spectrum rays to the eye of the observer (you!), and the eye then sends a message to your brain that this is a red object.

The human eye has receptor cells that perceive red, green and blue, which are also the primary colors of white light. (Not to be confused with artists' primary colors of red, yellow, blue!) The use of only three colors to reproduce thousands of colors is possible because the eyes are basically responsive to these three broad sections of the spectrum.

But what you see is not always what's really there! There are a lot of things that can affect the way you view color. For example, lighting conditions can change the way a color looks. A person who looks at color for a long period of time is going to experience retinal fatigue and the colors are not going to be perceived accurately. Furthermore, each person experiences the sensation of a given color differently, and therefore describes it differently. These are just some of the reasons that color communication standards are so important.

Methods for Reproducing Color: Additive and Subtractive Color Mixing

There are only two ways to reproduce color: additive and subtractive. Modern color reproduction involves both additive and subtractive color mixing processes. These two methods for reproducing color are based on the theory that all colors can be created using three primaries. Understanding these two systems and how they relate to each other provides the foundation for understanding color reproduction, both on the monitor and in print.

Additive Color When we mix red, green, and blue light to create colors, we are using the Additive Color Mixing process. It is called "additive" because we are starting with a black background and adding lights to create color. Combining all three colors will produce white. A color television, computer monitor, scanner, and stage lighting all demonstrate this process. By superimposing two additive primaries, we produce secondary colors that will become the primaries in subtractive color mixing: green + blue = cyan blue + red = magenta red + green = yellow red + green + blue = white

Subtractive Color Subtractive color mixing is the reverse of additive color mixing. We are starting with white and subtracting cyan, magenta, and yellow pigments to create black. Subtractive color mixing is used in the printing industry, since we are usually starting with white paper. CMY pigments absorb or "subtract" certain wavelengths of color from the white background, and allow others to be reflected: Cyan subtracts the red component of white light; magenta subtracts green light ; and yellow subtracts blue light. When we add subtractive colors, we end up with additive colors:

magenta + yellow = red yellow + cyan = green cyan + magenta = blue cyan + magenta + yellow = black

So, while additive colors are achieved by combining light, subtractive colors are achieved by combining color pigments, such as inks or dyes.

In the printing industry, we refer to printing as a four-color process: three primaries (CMY) plus black (K). Black ink is used to achieve a deeper black than could be made by mixing the three primaries. Color images are separated into four layers of halftone dots that vary in size and angle to create the illusion of different colors. Color Spaces

Color spaces are three-dimensional models that show you what colors are possible to use in your work. There are a number of different color spaces. It is important to distinguish between those color spaces that are based on color reproduction methods (gray spaces, RGB-based color spaces and CMY-based color spaces) and those that represent all the colors that we can see (device-independent color spaces and CIE color spaces).

Gray Spaces RGB-based Color Spaces CMY-based Color Spaces Device-Independent Color Spaces CIE Color Spaces

Gray Spaces

Gray spaces have a single component: black. Gray spaces are used for black-and-white (grayscale) display and printing.

RGB-based Color Spaces

Any color expressed in RGB space is some mixture of the primary colors red, green and blue. Most color displays use RGB-based color spaces.

Color spaces within the RGB space include HSV and HLS. These are transformations of the same space that allow colors to be described in terms more natural to an artist. HSV stands for hue, saturation, and value. HLS stands for hue, lightness, and saturation.

CMY-based Color Spaces

Most desktop color printers and the printing industry use CMY-based color spaces. There are two groups: CMY and CMYK. CMY is not very common and is used by low-end desktop color printers. CMYK adds black to compensate for the fact that cyan, magenta, and yellow cannot produce a true black when mixed together. So black is added to overprint these areas and give the image better contrast.

Device-Independent Color Spaces

Different devices have different gamuts, or ranges of colors, that they can produce. This means that RGB and CMY color spaces vary from monitor to monitor, from printer to printer. Thus they are called device-dependent color spaces.

When you convert from RGB on one device to CMYK on another, it can get tricky. This is where device-independent color spaces come in. As the title implies, device-independent color spaces are not dependent on any particular device, and are meant to be true representations of colors as perceived by the human eye. Device-independent color spaces are used for the interchange of color data from the space of one device to the color space of another device. They are a result of the research work done in 1931 by the Commission Internationale d'Eclairage (CIE) and for that reason are more commonly known as CIE-based color spaces.

CIE-based Color Spaces

The CIE color spaces form the foundation of device-independent color for color management. There are two types of CIE spaces: CIE L*a*b* and CIE Lch.

CIE L*a*b

This the most commonly used color space and is based on human perception of color; the three color receptors (red, green and blue) in the eye. This results in three sets of signals being sent to the brain: light or dark, red or green, and yellow or blue. They are opposing in that one receives a red signal or a green one, but not both. This opponent type color space is derived mathematically from the CIE values.

L is a measure of lightness of an object, and ranges from 0 (black) to 100 (white). a is a measure of redness (positive a) or greenness (negative a). b is a measure of yellowness (positive b) or blueness (negative b).

The coordinates a and b approach zero for neutral colors (white, grays and black). The higher the values for a and b, the more saturated the color is.

CIELCh

This color space is often referred to simply as LCh. The system is the same as the CIELab color space, except that it describes the location of a color in space by use of polar coordinates, rather than rectangular coordinates.

L is a measure of lightness of an object, ranging from 0 (black) to 100 (white)

C is a measure of chroma (saturation), and represents the distance from the neutral axis. h is a measure of hue and is represented as an angle ranging from 0° to 360°. Angles that range from 0° to 90° are reds, oranges and yellows. 90° to 180° are yellows, yellow-greens and greens. 180° to 270° are greens, cyans (blue-greens) and blues. From 270° to 360° are blues, purples, magentas, and return again to reds.