A good color means natural. Look carefully at a fistful of hair in the mirror. See how each strand has a different tone? Perhaps even different colors on one strand? These are the natural striations that Mother Nature gives each head of hair to work with natural lighting. A good "color job" must duplicate these natural highlights to be effective.
Plus few things offer the opportunity to change your look the way changing the shade of your tresses does. It lets you keep everything else virtually the same while still giving you the change you seek. Not a bad deal actually.