Almost every system is capable of displaying at least 65,000 colors simultaneously. Only two years ago I bought a Packard Bell computer, equipped with a rather dull Cirus Logic graphic card. Since the computer came with a 17-inch monitor, I had to compromise for only 256 colors. Adjusting it to a higher number of colors slowed down the computer, despite the screaming Pentium processor. My personal experience explains why some of your Web site visitors are surfing with 256 colors.
Most designers support the lowest common denominator to avoid image dithering.
The advantage of this approach is that it keeps the colors looking as they were intended, and makes the images look smoother. Nevertheless, it sacrifices quality that site visitors equipped with better video systems could enjoy. The colorDepth property of the screen object reveals the user's color bit depth. This property is very helpful if you're interested in providing the most suitable image for the user's color resolution. The conditional statement in the script checks if the browser supports the screen object.