Some web users may be unable to see color -- they may be blind (and use a screen reader program); they may be color-blind and unable to easily distinguish colors; or they could be using an access device, such as a cell phone, which does not display color. For this reason, the W3C's Web Accessibility Initiative recommends that you not rely upon color as the only way of conveying information.
Kynn says:
"Color is very
useful to those
who can see it"
This doesn't mean "don't use color" -- on the contrary, color is very useful to those who can see it, and will help make your page understandable. What it does mean is that color (and other presentational effects) should not be the only way you make something special. For example, instead of using a <span> and a color style to make something stand out, use the <strong> tag (which you can also style) so browsers that can't use the visual CSS color rule can at least know to emphasize that section -- perhaps by increasing the volume when reading the text out loud, for example.