Letter from Vincent van Gogh to Theo van Gogh, Saint-Rémy, 11 or 12 May 1890 [letter 633]: "...I am doing a canvas of roses with a light green back-ground and two canvases representing big bunches of violet irises, one lot against a pink background in which the effect is soft and harmonious because of the combination of greens, pinks, violets. On the other hand, the other violet bunch (ranging from carmine to pure Prussian blue) stands out against a startling citron background, with other yellow tones in the vase and the stand on which it rests, so it is an effect of tremendously disparate complementaries, which strengthen each other by their juxtaposition. These canvases will take a whole month to dry, but the attendant here will undertake to send them off after my departure. (http://www.tfsimon.com/auvers-sur-oise.html)