I used a reference photo of “Indian Canyons” park in Palm Springs, Ca to study shadow color.  The color of shadow on a surface is influenced by it’s local color, as well as the environment: objects facing the sky tend to have bluer shadows than shadows that don’t reflect the sky. A good area of the painting to observe is the top left quadrant.  The large boulder there has a striking blue shadow.  The color of the rock is near white (with some blue in it), but the reason the blue is so strong is the influence of the sky.  That sky color reflects into the shadows.  Compare that top shadow with the shadow on the left of the boulder, as it hits the river bed. There are several shadow colors there.  The side of the rock is a warm shadow,  it doesn’t face the sky plane, but instead has warm palm tree leaves to reflect. Yet the side also takes on an orange hue reflecting from the water below it.  That same cast shadow of the boulder’s left side hits the water, and and a smaller boulder behind.  That small rock is facing the sky at an angle, so has a deep blue shadow.The cast shadow on the water is more violet, as it is not getting as much sun as the top of the rock.