Web-safe palettes are usually limited to 216 colors to reserve the remaining colors for the operating system and browser. A good general rule is to use a Web-safe palette whenever possible. A popular way to use graphics on Web sites is to use transparency to make one or more elements stand out. Typically, a transparent background color around an irregularly shaped image will make that image stand out. Choosing a transparent color is an important decision.
Many people choose white to be transparent.
This decision can lead to problems, for although an image might be surrounded by white, it might also contain white. When you create a graphic that includes transparency, choose a transparent color that is confined to very specific areas that are not part of the image. Magenta is often used as a transparent color for it rarely appears in images. Another way to use transparency is for a background image, or watermark, over which you place text or other images. A common use of this technique is placing repeating images of a company logo as a partially transparent background.