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.
Thinning hair? Did you know you can make it appear thicker by the strategic use of color? By that I mean if you color your root line to deal with any regrowth you're also making the roots look a bit darker. Not only does this touch up your color it will also create the illusion of thicker hair. That's because roots that are a bit darker make your hair appear to be fuller and more dimensional.