BBC reports the findings of a study of the deterioration of the color yellow to brown in some of Vincent van Gogh's paintings:

"The researchers found that a change in the oxidation state of the element chromium (from chromium 6 to chromium 3) was linked to the darkening of chrome yellow paint."

For more information, read the complete article ("Van Gogh paintings 'degraded by UV-driven reaction'") on the BBC here.

Photo from the BBC website: Van Gogh's "Banks of the Seine", Oil on canvas, Paris, 1887, Van Gogh Museum, Amsterdam, The Netherlands