How many colors exist?
Well the human eye has been thought to percieve atleast 10 million colours, or even more. If you think about it, there is an endless amount as you can always add different ratios of paint together. EG 180 parts red 1 part green etc etc
There are 256 colors that are considered to be 'web safe' and should always appear the same tone...however, when you start to combine various shades, lightening, darkening, etc...I'd say there must be millions, if not billions :)