Today as I was painting on my 4'x5' canvas a painting of Pinnacle Peak in Scottsdale, I was asked about the colors in the rocks. "How do I know what color to paint them? I see so many different shades in your painting."
I started the painting by blocking in the large shapes in light and dark, which is the normal process. I then started in the shadow side looking at the warms and cools of the rocks. There are reasons for the variations in the warms, cools, and different values in both the shadow side and light side.
First, I'll explain by using the example of one, single rock. The color of the top side of the rock is altered by the color of the sky. In the case of the example here, it is a blue sky. So, the top of the rock in both the shadow and light side are cooler. The rock is warmer as it turns to the underside.
If the rock were on bright green grass, the underside would pick up some of the green color. The vertical side of the rock isn't affected as much and will be more of the local color of the rock. Now, onto the values, the top of the rock picks up more light from the sky, thus it is lightest even on the shadow side. The underside and crevices are, of course, darker.
The larger the crevice, the darker it is. Look for small nuances of temperature differences, warm or cool in the the space between the light and shadow. Most often I have seen a cool transition when looking at the cool light of the top of a rock gently curving into the shadow. I've seen a warm transition between the light and shadow when it is a sharper angle from light to shadow.
Besides just looking at color, you will want to think about your brushwork. When you see sharp edges and strong shapes, use strong brush marks. Don't overwork it. Place a brushstroke down and leave it! The gentle curves can be softened and smoothed out. Use different sizes of brushes and different strokes. And, use larger brushes as you move forward into space. This will help to give your painting more depth.
I also want to add, in this painting the composition is kept simple and strong by not varying my values a lot. In the shadow side, the range between the light and dark is very limited. The lights, the same way. That will make this painting read very well from a distance. I didn't break it up with too many shapes and too many lights within my darks. I hope this was helpful to you for painting strong rocks with form to them. Happy painting! Becky Joy