To express the love of two lovers by a marriage of two complementary colors, their mingling and their opposition, the mysterious vibrations of Kindred tones. To express the thought of a brow by the radiance of light tone against a somber background; to express hope by some star, the eagerness of a soul by a sunset radiance.

Further Letters of Vincent Van Gogh to His Brother, 1886-1889 (ed. 1929)