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There are so many color, style, font and image options when designing a website. Today's websites can contain anything from simple formatted ASCII text, to stereo audio recordings, full motion video, and high resolution, computer generated, real-time graphics and animation.
When the graphic web browser was developed it didn't even display colors -- it was developed on a NeXT computer which only produced grey-scale images.
Eventually browser software for PC's were developed to display the 16 color VGA Pallette.
Netscape's Navigator and Microsoft's Internet Explorer graphic browsers for systems with a minimun 640 x 480 VGA graphics display capability opened up the world of graphical web browsing to millions of users and reshaped the online world which was dominated by commercial information services and BBS's.
One standard method of defining colors in web pages is using the RGB / Hexidecimal Color Values in HTML tags. Modern browsers support many more colors, as shown in this chart of HTML Color Names & RGB Hex Values.