This book is the first complete translation of Erwin Schrödinger's work on colorimetry, of which he wrote about extensively (other versions from decades ago have appeared in partial and severely abridged form). He proposes that the geometry of color space is a projective geometry, rather than Euclidean. He also develops a dramatic conceptual shift in colorimetric methods, closely in detail and at length. Schrödinger concludes that the trichromatic (or Young-Helmholtz) theory of color and the opponent-process (or Hering) theory of color are really the same theory, or at least only trivially different. The resonance and importance of his bold concept of color space is emphasized for contemporary color theorists in this translation.