When sunlight travels through a semi-transparent material, the light becomes richly colored. Light that just bounces off the surface is fairly dull by comparison.
This “stained-glass-window effect" is called transmitted light, and you often see it when the sun shines through the green or yellow leaves of a tree. You might also see transmitted light when the sun backlights colored balloons, a sailboat’s spinnaker, or a translucent nylon awning.
This on-the-spot oil painting of a skunk cabbage plant is a study of transmitted light. The bright yellow-green area is much more intense than the other greens.
Here’s the picture again, with numbers superimposed in each area of the foliage to analyze what’s going on with the light and color:
1. Transmitted light, with intense chroma or saturation in the yellow-green range.
2. The leaf in shadow, facing downward. This is the darkest green, and would be even darker if it wasn't picking up reflected light from the adjacent leaf seen edge-on.
3. The leaf in shadow, facing upward. These ‘up-facing planes’ are blue-green, because they are picking up the blue light from the sky.
4. Sunlight reflecting off the top surface of the leaf. This is the highest tone or value, and the most textural, especially where it transitions to shadow. But the chroma is not very intense, because most of the light bounces off the waxy cuticle of the leaf.
When you are painting a faraway tree backlit by sunlight, it’s good to keep in mind these four conditions: transmitted, downfacing shadow, upfacing shadow and sunlit. These colors, visible in the skunk cabbage up close on a micro scale, are present here, too, mixed together like tiny pixels even if you can’t really see the component leaves.
The distant foliage is a composite of all four color elements, blended with the atmospheric effects. As you can see in this faraway view of autumn maples, there are more leaves shining with transmitted light at the lower left margin of the tree. The leaves in the central area are darker and duller because they’re lit by the cool skylight.