The blue irises in the foreground of Vincent van Gogh's Field with Irises near Arles,

painting in 1888, were found to have actually contained a red pigment that has faded

over time and were originally a purple colour

http://www.dailymail.co.uk/sciencetech/article-2877529/Van-Gogh-s-blue-irises-purple-Pigments-masterpieces-fading-bright-lights.html