Thread on color in painting part 2! How do we know how to confidently decide on colors in the infinite lighting situations that we can imagine? We’re going to dive deeper this time into how light works, and what determines the color we actually see when light reaches our eyes. 
First, materials. They all reflect the scene, just differently. Diffuse materials absorb/scatter some wavelengths beneath the surface while reflecting the rest, some metals reflect all of the light, and others are colored due to uneven absorption and emission by their electrons.
The more pure and strong the reflection of the light source, the further the shift in color towards it. The direction of hue shifting depends on the material. A yellowy-green grass will travel through the greens when reflecting the blue sky, while skin will go through the pinks.
By knowing the material, the environment, and 3D structure of what we are drawing, we can make a good guess as to the colors. All color is due to reflection. Imagine what each plane “sees.” When a surface is no longer “seeing” the light source, it turns into shadow.
In this first example, the planes being lit are “seeing” and reflecting a light source that is relatively colder than the indoor environment. In the second, with sunlight, the white light of the sun looks warm compared to the blue influence of the sky. (sunlight is not yellow!)
The color we see is actually energy. The sun produces it, and our eyes perceive a narrow range as color. High energy appears blue, lower energy red, and beyond those are ultraviolet and infrared. You might notice that the band of visible energy is similar to the color wheel.
The cells in our eyes detect the proportion of short (blue), medium (green), and long (red) wavelengths, then our brain mixes these into color. Visible light is not a wheel, but it can be conceived of as one with pinks being how our brain interprets the mixing of red and violet.
The colors we see depend not only on what wavelengths the object’s material can reflect, but also what wavelengths come from the light source. Sunlight is white light, or a mix of all of the wavelengths.
The sky scatters short wavelength blue light from the sun (Rayleigh scattering). Planes in the shadow here “see” mostly the sky. The shadow on the ground sees the plane of the cheek on the far side of the face, mixing with the sky in the shadow color.
Blue light from the sky results in the shadow areas reflecting cool light, but that strong bounce light warms up the downward facing planes since those see the ground. Areas that bounce more than once have increased saturation, or purity, like a mirror reflecting multiple times.
If we have an object that reflects blue light, which would appear blue under white light, under a light source that only emits red light it would appear black since it is absorbing all of the energy.
For more on reflected light, see my previous thread on shadows!
Stay tuned for part 3, and if you've enjoyed this, check out my ebook! It has all of my previous tutorials greatly expanded upon and with lots of bonuses like a glossary, recommended reading, and all the artists names right there with the examples! gumroad.com/l/YPtf
Artist credit: Isaac Levitan, John Singer Sargent, Leopold Carl Mueller, Konstantin Makovsky, Anders Zorn
