HTML and CSS Reference
G.3. 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 vari-
ants of red, six of green, and six of blue that are combined in all possible
ways to create these 216 (6 x 6 x 6) colors. These variants have decim-
al 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
(dark cyan) and 999999 (medium gray) exist directly in the color map
and won't be converted or dithered.
Keep in mind that many of the extended color names are not in the
standard color map and will be converted or dithered to a (hopefully)
similar color. Using color names, while convenient, does not guarantee
that the browser will use the desired color.
When creating images, try to use colors in the standard color map. When
selecting colors for text, links, or backgrounds, make sure you select col-