This book presents the most complete translation to date of Erwin Schrd̲inger?s work on colorimetry. In his work Schrd̲inger proposed a projective geometry of color space, rather than a Euclidean line-element. He also proposed new (at the time) colorimetric methods ? in detail and at length - which represented a dramatic conceptual shift in colorimetry. Schrd̲inger shows how the trichromatic (or Young-Helmholtz) theory of color and the opponent-process (or Hering) theory of color are formally the same theory, or at least only trivially different. These translations of Schrd̲inger?s bold concepts for color space have a fresh resonance and importance for contemporary color theory.