HTML & XHTML: The Definitive Guide, 6th Edition

The Standard Color Map

Supporting hundreds of color names and millions of RGB triples is nice, but the reality is that a large (albeit shrinking) population of users can display only 256 colors on their systems. When confronted with a color not defined in this set of 256, the browser has two choices: convert the color to one of the existing colors, or dither the color using the available colors in the color map.

Conversion is easy; the color is compared to all the other colors in the color map and is replaced by the closest color found. Dithering is more difficult. Using two or more colors in the color map, the errant color is approximated by mixing different ratios of the available colors. When you view them up close, you’ll see a pattern of alternating pixels using the available colors. At a distance, the pixels blend to form a color close to the original color.

In general, your images will look best if you can avoid both conversion and dithering. Conversion will make your colors appear “off”; dithering makes them look fuzzy. How to avoid these problems? Easy: use colors in the standard color map when creating your images.

The standard color map actually has 216 values in it. There are six variants of red, six of green, and six of blue that are combined in all possible ways to create these 216 (6 × 6 × 6) colors. These variants have decimal brightness values of 0, 51, 102, 153, 204, and 255, corresponding to hexadecimal values of 00, 33, 66, 99, CC, and FF. Colors such as 003333 ...