Even though a graphic having a large number of colors is beautiful, the tradeoff is a large file size. Reducing the number of colors within a graphic will make the file size smaller. So, there are bandwidth issues that make smaller file size graphics better than their bulky counterparts.
There is another important reason to keep the number of colors low. Certain viewers have either their screen or their browser set to receive only 256 colors. When making graphics, this color number barrier must be taken into consideration.
It is suggested that designers create their graphics using browser safe colors. Most graphic design software programs have a setting that only enable such colors to be used, making them appear to the viewer just as they were designed.