Values, the dark and light shapes, are what gives the composition of a painting weight and shape. The right value of a color gives the color richness. Everybody sees color differently, we all have our own color sense that we try to develop in our painting but values are what make a painting work or not. We can use a higher or lower key of values ( making the range of values in a painting darker or lighter) but the relationship beween the darks and lights have to be the same. So practicing getting the right values in a painting is important and sometimes hard when you are also dealing with color. A good exercise is to do a small value study before doing a larger painting. You can use black and white or to give the study a little more of a warm temperature you can use raw umber and white. Keep the values simple, break the values down to a range of 5 or 6, 2 in the light, a halftone and 2 in the dark. Just like shapes and color keep the values simple.