There are many different browsers and you want your site to be seen by your customers how you set it up. Some colours are not rendered (produced) exactly the same in all browsers. This is because most colours are dithered colors - this means they are produced from a blend of two colours. How do you avoid this problem? Simple - don't use blended colours .. just use pure colors that every browser and monitor reproduces the same. These are called Web Safe Colors. Because of differences between Netsafe and Internet Explorer we are left with 216 useable colors out of a possible 255 pure colors.
Hope that helps, Carl (RoseWeb)