Four colours and the visual separation of adjacent areas: lessons from mapping and ancient paintings

Abstract

That four colours were sufficient to differentiate adjacent countries on a map was a 19th century conjecture which has taken 150 years to prove mathematically. In a different sphere, and two and a half millennia earlier in Ancient Greece, many painters including Apelles favoured the use of four colours. A story recounted by Pliny in which three or four colours were used to differentiate thin lines, however, serves to link these seemingly disparate observations of the mathematical and the artistic. Furthermore, the use of such few colours to achieve differentiation of adjacent areas can thus be seen to date back to classical times, if not beyond.