I use a 17 inch monitor and have set my resolution to 1024x784. People who have bigger monitors may use other resolutions. When you make a page to fit one resolution, it looks different in another. People using the 640x480 setting may have to scroll to see your whole page unless you designed the page to fit that resolution. We're just getting starting.
Computers have different color settings.
There are many reasons. It is not available on some computers. In any case the more colors you show the more memory it takes. Older computers came set to 16 color. Probably the most common setting today is 256 color.