|Twilight in the Wilderness, Frederick Edwin Church, 1860|
As matter interacts with photons on the journey to Earth, the wavelength dictates each color's reaction. The sky is blue because the paths of the short blue waves are more prone to collision. The blue light scatters easily in atmospheric dust and moisture. Church used a pigment called Cerulean, which was developed for skies, despite being a little too green. Interestingly, if our vision was more accurate, the sky would be violet. Our brain makes it blue. Our art history makes it turquoise.
Clouds are wonderful paint and color subjects. They're generally white, because the water vapor that composes them diffracts sunlight and makes them glow. When clouds are dense with vapor, on the brink of rain, less light escapes and they turn gray. At twilight only the photons on longer waves reach the clouds, lighting them up red like fire. Of course that's a bad analogy because we tend to see fire at relatively low temperatures— reds and yellows are called 'warm colors' while blues and greens are called 'cool colors,' despite the fact that blue light is hotter than red.
The remaining colors in the painting are desaturated but mostly green: the predominant color of plant-life. Terra Verte is a likely pigment. Plants live through a process called photosynthesis: photons from sunlight provide the energy to chemically bond carbon dioxide to water, forming carbohydrates. The subroutine of translating light energy to electric energy occurs in the photosynthetic reaction center: a mesh of proteins and pigments. Chlorophyll is a typical pigment— chlorophyll absorbs the blue and red light, and reflects the green. If a plant's leaf is the color of Terra Verte, that is its resonant frequency. The leaf system stores vibrational energy at that frequency, so the Terra Verte photons are amplified upon reflection. A well-informed painter might also know Terra Verte has been used as an undertone for flesh since the 11th Century, and can subtly anthropomorphize a landscape.