© 2021 selection and editorial matter, Derek H. Brown and Fiona Macpherson; individual chapters, the contributors. Colour vision has been studied systematically from a variety of points of view since the nineteenth century. The science, the readers discuss draws on optics, psychology, neuroscience, neurology, ophthalmology, and biology. Only a very small segment of the total electromagnetic spectrum is relevant to most questions in colour science because the receptors in the eye only respond directly to a narrow range of wavelengths. Since colour vision requires the ability to distinguish between lights with different wavelengths, that means that colour vision requires contributions from at least two types of photoreceptors that differ in their spectral sensitivity. The three cone-types are morphologically indistinguishable, and although their existence was inferred in the nineteenth century in order to explain the observed characteristics of human colour vision, it was only in the late twentieth century that direct measurements of their spectral sensitivities were made, and the light absorbing photopigments.