“Van Gogh and Seurat occasionally spoke of ‘unnameable’ or ‘indefinable’ colours, which suggests how much the spread of colour-order systems in the late nineteenth century had led to the expectation that colours could indeed be defined. . . . The indefinable colours were of special interest to Gauguin, who was careful to avoid the strong contrasts of his friend Van Gogh, and who specifically exploited secondary and tertiary hues in the interest of what he called ‘enigma.’”

—John Gage, from Color in Art, 2006.