The nature of color is an open philosophical and scientific question. In this work I develop a physicalist relationist theory of color. So far, attempts to identify color as a physical property of objects have not been convincing because no physical property used by scientists seems to be well correlated with color sensations. I define a new physical property which I call transformance and show that transformance is 100% correlated with color sensations. Intuitively, transformance is a very general abstract physical (...) property that describes how a system transforms or modifies light or information that characterizes light. It turns out that transformance is a relational property of objects like velocity and weight. Transformance is related to surface reflectance as weight is related to mass. Transformance is a much better candidate to be color than surface reflectance because it is 100% correlated with color sensations, precisely models surround effects and fully explains all issues that relate to perceptual variation or the lack thereof. Several concrete examples are provided that show how the transformance of different systems should be modeled. After defining transformance, I defend the theory that color is transformance against possible objections while contrasting my theory with the theory that color is surface reflectance. I then discuss the relations between color and color sensation and the more general objections to any physicalist theory of color. The final part of the thesis deals with epistemological issues related to my theory of color including two apparent paradoxes that I believe every theory of color must answer: 1) How is it that human beings get along so well with an erroneous theory of color? 2) How can science, which is based on the common sense theory of color, conclude that the common sense theory is wrong? I show that my theory provides answers to these questions. (shrink)