First, we have to understand the difference between spectral (additive) colors and pigments (subtractive colors). Spectral colors are the light waves emitted from a source such as the sun, while pigment colors are mixed to affect what we see when light reflects off an object. Our sun emits white light, which is a mix of all the colors of light mixed together. When this light passes through a prism, such as water drops in the atmosphere after a rainstorm, the light is scattered into its component wavelengths, which our eyes interpret as different colors: red, orange, yellow, green, blue, indigo, and violet.
Our sun emits all these colors on its own, which is why we’re able to see white objects. Other light sources such as computer monitors use an additive process to create the colors we see. For example, the yellow and orange stripes you see coming from the prism above aren’t actually orange or yellow at all. Your monitor is lighting up red and green pixels that are so close together than your eyes blend them together to make a new color. In fact, every single color on your monitor is made up of a mix of red, green, and blue light in different concentrations. Additive light is also used in theatrical lighting. While it may appear that actors on stage are under white light, this is rarely the case because pure white light is very harsh and, due to the limitations of the filaments in the light bulbs used, starts turning orange-y if the lights are dimmed at all. For “white” light, different colored gels are placed on the light fixtures and the pools of light on the stage are arranged so that the different colors remix to make white. Usually, this means paired amber and blue lights aimed at the same spots, repeated enough times to cover the stage. In additive light, white is the presence of all colors, while black is the absence of color. There is no way to project true gray light since putting a gray filter on a light screens out every wavelength equally to dim the light; this is why nighttime scenes are typically lit in blues.
When most of us think about how colors are mixed, we’re thinking of pigments. Red, yellow, and blue are the primary colors that mix in equal proportions to form the secondary colors orange, green, and purple, and then in different proportions form countless other colors. Mix all of three primaries together, you get black; take them all away, white. Adding black pigment to any color makes a shade (think midnight blue, or brown as a shade of orange), while adding white pigment (or thinning a medium like watercolors with extra water and applying it to a white surface) makes a tint (pink being the one that pops to mind first, but really all pastels). Pigments absorb certain wavelengths of light and reflect others; it’s that reflection that our eyes pick up and read as a particular color. This method of color mixing is used in painting, color photo development, and other art forms.
In printing, however, a different subtractive process is used to apply pigments to paper or other media. In contrast to the red, yellow, blue mixing we just discussed, printing generally uses CMYK (cyan, magenta, yellow, and black) in different concentrations. Technically, cyan, magenta, and yellow form black when all mixed together, but it’s much cheaper to have a separate print cartridge of black ink rather than deplete all three color cartridges. This process only works properly on white paper since it’s impossible to print the color white using CMYK; white spaces are left unprinted so the background color shows.
Krulwich is correct when he states that “pink is not a real wavelength of light,” but then again, neither are most colors. Pink is how our eyes interpret a mixture of wavelengths that include red, violet, and maybe some indigo or blue if it’s a really dark fuchsia or some orange or yellow for a peachy pink. There’s no purple wavelength either since it’s a mix of red and blue pigment; the light in the rainbow is violet. But as we’ve seen, pure wavelengths of light are only one tiny way of defining the colors around us.