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Color "depth" is defined by the number of bits per pixel that can be displayed on a computer screen. Data is stored in bits. Each bit represents two colors because it has a value of 0 or 1. The more bits per pixel, the more colors that can be displayed. Examples of color depth are shown in the following table:

Determining Color Depth

Since each bit represents 2 colors, it is easy to work out the number of colors for the various color depths. The number of possible colors would be 2 to the power of the number of bits per pixel:

A color depth of 4 bits would be 2 times itself 4 times:
2 x 2 x 2 x 2 = 16 colors

A color depth of of 8 bits would be 2 times itself 8 times:
2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 = 256 colors.

A color depth of of 24 bits would be 2 times itself 24 times:
2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 x
2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 = 16,777,216 colors

True Color

Images are known as "True Color" where each pixel is defined in terms of its actual RGB or CMYK values. Every pixel in a a true color image has 256 possible values for each of it's red, green or blue components (in the RGB model) or cyan, magenta, yellow and black (in the CMYK model). Because there are 256 possible values for each RGB or CMYK component, then RGB true color would have a 24-bit color depth and CMYK true color would have a 32-bit color depth. There are millions of possible colors for each pixel in a true color image. That's why it is called "True Color".

RGB images are derived from the 3 primary colors of red, green and blue. In 24-bit RGB color, each red, green and blue component is 8 bits long and has 256 variations in intensity. These variations are represented in a scale of values ranging from 0 to 255 with 0 having the least intensity and 255 having the greatest. When the 3 components are combined there are 256 x 256 x 256 possible combinations or 16,777,216 possible colors.

For example, white would be composed of maximum intensity of red, green and blue light (R=255 G=255 B=255) and black would be composed of zero intensity of red, green and blue light (R=0 G=0 B=0). Cyan would be composed of maximum intensity of blue and green light and zero intensity of red light (R=0 B=255 G=255). Magenta would be composed of maximum intensity of red and blue light and zero intensity of green light (R=255 G=0 B=255):

True Color and The CMYK Color Model

Images using the CMYK color model are also true color. CMYK images are derived from the 3 primary colors of cyan, magenta and yellow plus black. In 32-bit CMYK color, each cyan, magenta, yellow and black component is also 8 bits long and has 256 variations in intensity. Each pixel in a 32-bit CMYK image is one of 256 x 256 x 256 possible colors x 256 variations of black. A mix of 100% each of cyan, magenta and yellow produces black, so the black component is extra. Even though there are more bits per pixel in the CMYK model, in reality it has a smaller color "space" or gamut than RGB . Many graphics programs support both color models.

Indexed Color

Images which do not define colors in terms of their actual RGB or CMYK values and which derive its colors from a "palette" are known as "Indexed Color". The color palette of an indexed color image has a fixed number of colors. Because the palette is limited to a maximum of 256 colors, it is not possible for an image to look as realistic as it can using RGB or CMYK. Hence, they are not true color. This type of color is known as "Indexed Color" because colors in the palette are referenced by index numbers which are used by the computer to identify each color. Some file formats restrict the number of colors to fewer than 256 . The GIF format is one such format and has a color depth of 8 bits per pixel or less. GIF files use indexed color and allow a maximum of 256 colors. TIFF files can be stored as indexed color or true color. A sample palette is shown below:

Windows 16 Color Palette with Index Numbers

Grayscale Images

Grayscale images have a maximum color depth of 8 bits. The reason for this can also be worked out easily. When defining shades of gray in terms of RGB, each of the 3 red, green and blue components must be equal to each other. Examples of grays are R=192 G=192 B=192, and R=128 G=128 B=128. Since all three components must be equal for any shade of gray there are only 256 possible combinations. Thus, grayscale images have a maximum color depth of 8 bits. A complete 256 color grayscale palette is shown in the sample images below.

It is possible to create a grayscale image with a 4-bit color depth or less. Some software packages allow the user to reduce the number of colors in the palette from 256 to 16 or 2, though one would hardly classify a 2-color image as grayscale.

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Note: The palette images below have actual color values in them. If you wish you can download them and save them to your hard disk.

Sample Images	Palettes
1 bit color:	2 color palette: black and white
1 bit color:	2 color palette: blue and white
4 bit color:	16 color palette: Windows 16 color palette used for screen graphics
8 bit color:	256 color palette:
8 bit grayscale:	256 shade grayscale palette:
24 bit color:	(No palette used)

Spot Color vs. Process Color

So far, all that has been described in this article applies to computer displays. But what about printing? What if you want to print a job without using process color? What if you want to print a two or three-color job? What if you want to use a special color ink such as on a company logo? In this case use "spot color". Spot color is the use of pre-mixed colors rather than by combining color from cyan, magenta, yellow and black.

To ensure consistency of color the printing industry has adopted standards for spot color. There are several spot color systems but the most widely used is the PANTONE® system. Pre-mixed colors are selected from swatch books. These same color swatches appear in graphics programs.

When printing a spot color job, it is best to use PANTONE colors to ensure consistency of color. However, you may want to mix your own color for a company logo. In this case have the printer contact his ink sales representative and have the ink company formulate the color for you. This service should cost little or nothing more than the ink itself and can go a long way to establishing that "special image" for your clients.

Many companies use a combination of four color process and spot color in the printing of their product catalogs and sales brochures. The photographs of the products themselves are printed using four color process and the company logo with a standard PANTONE spot color or custom color. This would be a 5-color job. There are many variations on this theme but the basic printing methods are spot and process.