If you specify one of these colors, it is a good idea to specify all of the colors. This is because you have no control over the default colors a browser uses. For example, if you specify black text and no color for the background, the text will be invisible when viewed with a browser that has its background color set to black.

Color palette for graphics:
You can specify one of two different color palettes to use when converting graphics. The Mac palette contains 256 colors and will produce images that look great on a Mac. Unfortunately, Windows computers can only display 216 of those 256 colors. To restrict the palette to the 216 colors common to both Mac and Windows, you should click on the "Universal" palette. If you are interested in learning more about using colors in Web pages, there is a good discussion of it at:

http://www.connect.hawaii.com/hc/webmasters/Netscape.colors.html

Color dithering
Color dithering is a technique used to display colors that are not available in the current palette. It alternately changes the color of adjacent pixels to approximate the color that isn't in the palette. Using color dithering will match colors well, but the dithering may make the image look grainy. If color matching is more important to you than a little graininess, you should choose to use dithering. If the graphics in your document use only the colors of the selected palette, color dithering will have no effect.