I have been wondering where I got the link to look at Jerry Lebo's blog. I can't remember or find it. He works full time and tries to get in sixty minutes a day on his art work. He is a wonderful painter and writes instruction articles very well. This one about value and color is great for me. I always need help with that part of painting.
Sixty Minute Artist: "Relative" Value and ColorAt the workshop with
Bob Rohm at the end of October, he stressed value changes also. One of his tips for pastel painters was to pick out about 3 values of each color that you are thinking of using in the painting so you remember to use them. This would be a light, medium and dark green for example. Susan Carlin was talking about Helen Van Wyk in one of her comments and Helen uses 5 main values in a painting.
The body color, which is the main color of the item.
The body shadow, where the form turns.
The cast shadow, where something actually casts a shadow.
The reflected light.
The highlight.
To clarify this here is a snippet of one of my tomato paintings.

Original Painting

Helen Van Wyk's five values

My painting corrected with darker body shadow and darker shadow.
In my original painting, I showed the form of the tomato mostly with color. You can see that the painting would be more dramatic with the stronger value changes. If you can't see the difference in the first and last, squint a little. From across the room it would make a big difference. Maybe I am learning something over the years. :)

Maybe you can see the difference better in black and white.