There is something unique about the color purple: Our brain makes it up. So you might just call purple a pigment of our imagination. It’s also a fascinating example of how the brain creates something beautiful when faced with a systems error. To understand where purple comes from, we need to know how our eyes and brain work together to perceive color. And that all begins with light. Light is another term for electromagnetic radiation. Most comes from the sun and travels to Earth in waves. There are many different types of light, which scientists group based on the lengths...