Where I live, on many hiking trails you can’t count on reliable cell service. While smart phones and GPS are great, I tend to always carry a map with me in case I get lost. Glancing at the trail map, it’s another source of color with shades of green representing elevation and colorful lines for the different trails. The colors I see are the result of a different mechanism than what we discussed in the previous chapter. The mixing of colors using the CIE diagram as we have described in the previous chapter is called additive mixing. In contrast there is a related phenomenon called subtractive mixing. While additive mixing is highly predictable, subtractive mixing is much less predictable. It really depends upon the details of the color spectrum. This is particularly important for pigments such as those used in paints. It’s also the mechanism for the colorful ink printed in the figure of this book in your hands.