What Determines the Colors We See?

Is color a property of the external world? Are the surfaces in the external world labeled by colors for the eye and brain to see passively? Or is color a property of the brain, which actively constructs the colors of the surfaces from the information received from the external world? In other words, is color the cause or the consequence of our visual perception? These questions are at the interface of biology and physics, and even of philosophy. We will address them later by answering what determines the color we see.

Due to these questions, we need be careful about how the word ``color'' is used. In the following, the word color may be used in two different ways: to indicate the energy spectral distribution $L(\lambda)$ of a light signal the eye receives (better described by the word light), or to represent the corresponding visual perception, the color we see. This distinction is necessary simply because spectral distribution $L(\lambda)$ is not the only factor that determines what color we perceive, an important point to be discussed later.