From the Tate gallery site:

This work was made during a holiday at the fishing port of Collioure in the south of France in 1905, when Matisse and Derain painted portraits of each other. Under Matisse’s influence, Derain had begun to use strong, non-naturalistic colours, applied in small separate brushstrokes, to convey the sensations of light and shade. Their radical use of colour led critics described them and their associates as ‘Fauves’ or wild beasts, and ‘Fauvism’ became an important parallel to the rise of Expressionism in Germany.