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Color Depth

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:

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.

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.

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