I wrote a paper a few years ago in an art history class about the use of blue pigments on Egyptian sarcophagi. The problem with the organic pigments that were used is that they discolour over time, so what now appears to be green, black, or even brown was once a bright blue colour.
An article that I read on this subject largely accredited the discolouration with the choice of medium mixed in with the pigments. As I know there are debates and preferences still on the choice of medium used for oil paints (I’ve heard that linseed oil is going out of style as it tends to yellow over time), I think this is an interesting point as we see that these artistic choices are once that painters have always struggled with.
The darkening and discolouration of the blue paint on the Egyptian artifacts was concluded to be due to the organic compounds mixed in with the paint – chemical analysis shows that beeswax, gum arabic, and plant materials including glucose were variously present in the formulas. After oxidizing for thousands of years, these substances changed the original appearance of the paint.
This sort of information is critical for those involved in art restoration and artifact preservation. The objects used in this study were all made of wood, and the authors acknowledge that the discolouration of these pigments on other surfaces would likely be from other circumstances.
Daniels, Vincent, Rebecca Stacey, and Andrew Middleton. “The Blackening of Paint Containing Egyptian Blue.” Studies in Conservation 49.4 (2004): 217-30. JSTOR. Web. 15 Mar. 2013. <http://www.jstor.org/stable/25487699>.