The history and usage of pigments are intimately related to the development of technology, physics and chemistry. This chapter introduces some of these technical details, including the nature of light and how it interacts with the human eye and colored substances, and how pigments as colored substances fit in. You can easily skip the technical details of this chapter without detracting from the joy of the pigment journey you are about to embark upon.

If you looked up into Earth's big blue sky on 19 October 2017, you may have sighted a potential visitor from far beyond the solar system. The object, shaped like a cigar and dubbed 1I/2017 U1, was sighted just passing through our astronomical neighborhood. Harvard astrophysicist Avi Loeb lent credence to this possible “social call” by aliens in a blog post for Scientific American, later transformed into a book.^2,3 If the inhabitants of 1I/2017 U1 had decided to stop off to say “hello,” and if their point of origin were a planet tied to the closest red giant, Gamma Crucis, they would have stumbled out of their spaceship functionally blind.

Why is this so? Why couldn't these aliens see non-red objects? We need light to see and there are different forms of light – the light coming from our sun is different from the light emitted by Gamma Crucis. What is the nature of this difference? To find out, we need to back up a bit, examine the nature of light, and then its relationship to color.

Now, getting back to our visitors from Gamma Crucis, a star that emits a range of energy that maximizes in the IR, invisible to human eyes. But the evolutionary pattern of the light receptor cells in our visitors' eyes would have necessarily maxed out in that range (because that's where all the light is on their planet) to find food or avoid becoming someone else's dinner. Our human eyes, for the same reason – by dint of biological evolution – are sensitive to the energy range of the sun's rays of maximal intensity that actually manages to penetrate our atmosphere – namely the red, orange, yellow, green, blue and violet radiation.^8,9 Hence we call this range, or spectrum of energies, the visible region (for us!). If we traveled to our Gamma Crucis visitors' planet, it would be our turn to be blind – because their solar spectral irradiance curve would maximize in the infrared region.¹⁰

The color mixing circles shown in Figure 1.3 provide the rationale for complementary colors. Figure 1.3 (left) depicts additive color mixing, which is the mixing of lights.¹⁴ Each of the three major regions of the visible spectrum can be roughly divided into the red (600–700 nm), the green (500–600 nm) and the blue (400–500 nm) regions. If all three colored lights are added together, they will produce white light, as seen in the center of the three overlapping circles (which is why sunlight appears white: it contains all of the spectral colors). We call red, green and blue the additive primary colors. When neighboring circles overlap, they produce the complementary colors: green + blue = cyan (a turquoise color); blue + red = magenta (sometimes called purple); red + green = yellow. So the complementary colors are yellow (to blue), magenta (to green) and cyan (to red). Now can you see why staring at a magenta star overwhelmed your red and blue sensitive cones for a while, and you could only see green for a time. In Figure 1.3 (right), we see the subtractive color system. Here, the three subtractive primary colors are yellow, magenta and cyan; their respective complements, shown in the overlapping neighboring circles are, respectively, blue (magenta + cyan), green (cyan + yellow) and red (yellow + magenta). A combination of all three subtractive primaries yields black. This is the system used in four-color printing.¹⁵ Check the colors of your laser printer cartridges – they correspond to the subtractive primaries plus black. We use this system when mixing pigments in a work of art: when complementary colors are placed adjacent to one another in a painting, they enhance one another producing a brightening effect; adjacent non-complementary colors, on the contrary, produce a duller effect (Chapter 13.4). Additive color mixing is good for adding lights such as theatre lights and the pixels on your television screen; it is also the principle behind photonic and structural colors (Chapter 16.3).