Why did a tapestry dye chemist change how painters use colour?

A chemist in charge of a tapestry dye house gave painters one of their sharpest tools. In 1824 Michel Eugène Chevreul was named director of the dye works at the Gobelins Manufactory in Paris, in response to complaints about technical inadequacies. He found that some dyes really were poor, but that the most criticised one, a black, was first-rate. Yet fabrics dyed with it looked weak and reddish when surrounded by deep blues or purples.

The black had not changed; its neighbour had. Chevreul called the effect simultaneous contrast: a colour seems to shift toward the complementary of whatever sits next to it, in both hue and darkness. Complementary colours are pairs that cancel each other's hue when mixed as light. A yellow fabric on a blue background appears tinted orange, because orange is the complement of blue.

Painters took notice. Chevreul published his research in 1839, and it influenced Impressionist and Neo-Impressionist painting, notably the Pointillism of Seurat and Signac, with its tiny juxtapositions of complementary colours. Van Gogh made lavish use of complementaries so that each intensified the other. The lesson for reading a painting is simple: a colour is never judged alone. Put opposites side by side, and by Chevreul's law each pushes the other further from itself.