Seeing color with three cones We’ve come up with a bunch of different tools for describing lights, describing photopigments, and describing how a photopigment responds to a specific light. These tools are simple, but quite useful. They give us a way to describe a lot of things that can happen to light independent of the visual system, and also to describe what happens when light does encounter the visual system at the retina. Let’s start by talking about that first set of circumstances: How can we use emission and absorption spectra to model different things that can happen to light out in the world, before it gets to our eye? One good way to see how these lists are useful for describing light in the environment is to consider various ways that colors can be mixed together. You almost certainly know a few things about mixing colors together. Suppose you scribble some red crayon a piece of paper and then scribble some blue crayon on top of that. I’m sure you can predict that the