Graphics > Color Basics > Color - What's It All About?

Introduction
Technical writers need not be expert in the physics of light or the physiology of the eye and nervous system but an understanding of the fundamentals of vision and imaging will assist in developing illustrations and graphics important to the success of a document.

The human eye, and current high-quality digital cameras, scanners, and other image acquisition devices capture millions of colors. As a result, imaging professionals have invented models for specifying colors, most of which can be mapped to a 2D, 3D, or 4D coordinate system similar to a Cartesian coordinate system. These models are called color spaces and the color components are called color channels.

Red, Green, Blue (RGB) Color Spaces
RGB is created by mapping the colors red, green, and blue onto a 3-D Cartesian coordinate system. The result is the color cube shown in Figure 1. The origin point of the coordinate system is black. This is where the red, green, and blue (RGB) color components are all 0.0. The diagonally opposite corner of the cube is white, where the RGB color components are at their maximum value. that is, all color values are restricted to the range of zero to one inclusive. So black is (0.0, 0.0, 0.0), and white is (1.0, 1.0, 1.0). The primary colors are red, green and blue.

Figure 1: RGB Color Cube

Hue, Saturation, And Value (HSV) Color Spaces
Hue, saturation, and value (HSV) are often used by artists. Hue is what we normally think of as color. Saturation is the amount of gray, white, or black that is mixed into the color. Zero saturation indicates no hue, just gray scale. A color in HSV space is specified by stating a hue angle, the saturation level, and the value level. A hue angle of zero is red. HSV color spaces can be device dependent or device independent.

Hue, Lightness, And Saturation (HLS) Color Spaces
HLS – hue, lightness, saturation – is also widely used by artists. Its color components are hue, lightness, and saturation.

Hue has the same meaning as the HSV model. Lightness is the amount of black or white in a color. Increasing lightness adds white to the hue. Decreasing lightness adds black to the hue. Saturation is a measure of the "purity" of a hue. As saturation is decreased, the hue becomes increasingly gray; saturation value of zero results in a gray-scale value. HLS color spaces can be device dependent or device independent.

Cyan, Magenta, Yellow (CMY) And Cyan, Magenta, Yellow & Black (CMYK) Color Spaces
The CMY and CMYK are often used in color printing. CMY uses cyan, magenta, and yellow (CMY) as primary colors; red, green, and blue are the secondary colors. Figure 2 shows color representations of CMY. The CMY color is normalized and subtractive, i.e. white is at (0.0, 0.0, 0.0) and black is at (1.0, 1.0, 1.0).

Figure 2: CMYK Color Cube

The CMY color space is subtractive. Therefore, white is at (0.0, 0.0, 0.0) and black is at (1.0, 1.0, 1.0). If you start with white and subtract no colors, you get white. If you start with white and subtract all colors equally, you get black.

The CMYK is a variation on the CMY model. It adds black (Cyan, Magenta, Yellow, and blacK). The CMYK color space closes the gap between theory and practice. In theory, the extra black component is not needed, but experience with various types of inks and papers has shown that when equal components of cyan, magenta, and yellow inks are mixed, the result is usually a dark brown, not black. Adding black ink to the mix solves this problem. The CMY and CMYK colors spaces can be device independent, but most often they are used in reference to a specific device.