short-lived pigments capable of fading or changing, especially with exposure to light, to atmospheric pollution, or when mixed with certain substances.
http://www.bluemoonwebdesign.com/art-glossary.asp
Pigments and dyes that fade or change color upon exposure to light (particularly ultraviolet) and/or heat. Artists and printers of reproduced art are not always sensitive as to whether or not the inks or pigments they use are fugitive. ...
http://www.framersworkshop.net/terms.html