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.
Coloring tip: It's a fact. The longer your hair is the older the ends are. Okay no news there. What may be news is that this makes them more porous and open to accepting hair dye. Which is why you want to get to them last if coloring at home. Otherwise they may end up darker than the rest of your strands leaving you with a final result that is well, interesting looking. Just not in a cute and sexy way.