All the Colours of the Rainbow: Robert Grosseteste’s Three-Dimensional Colour Space

The De colore—Robert Grosseteste’s treatise on colour—is a dense text of fewer than 400 words but one that presents a number of deep puzzles and challenges. It has received relatively little study and yet forms an important element within Grosseteste’s ‘scientific’ canon. The treatise presents an abstract account of the perceptual variation in colour, linked to specific properties of light and matter. It introduces new terminology in a way that assumes very tight definitions but that permits no straightforward translation or interpretation. We present an analysis of Grosseteste’s account of colour that draws upon his later treatise—the De iride—on the rainbow, which explicitly links the terminology of the De colore to properties of natural rainbows. A possible mapping between perceptual variation in colour and Grosseteste’s terminology is revealed by considering the physical constraints on colour production in rainbows, and the biological and psychological constraints on human colour perception.