Again: value does the work; color gets the credit!

In my classes we often discuss and practice value studies.  Value is the difference between dark and light.  While some paintings work very well with minor value change (i.e. little difference in darkness or lightness regardless of the color), paintings with a variety of value levels (some strong light and dark tones/values along with medium values) tend to be more exciting and energetic.  
(on laptops: tilt your screen to see more clearly).

Minor Value Difference

This top image has mostly medium values - mo really strong darks and lights. It's pleasant but a bit uninteresting.

Strong Value Difference

The middle image shows more value changes: strong darks, bright lights and several varying middle range values.

      

Color w Value changes

This final image has translated the multi-value middle image into color.  

   
Many artists will paint their piece first in gray scale values (top two images) to work out the values before adding color.  Then they can mix their color to the value on their underpainting without having to think too much about it.

    
I don't work that way usually though it's a great exercise to train your eye for value differences.  I challenge those of you who paint to try a couple paintings (small is easiest) doing a value underpainting then add your color at the end.  Let me know how that works.