Oil painting medium changes the color of the paint – Jos van Riswick
http://www.postcardfromholland.com (more info and 15 hour instructional video download)
Demonstration of oil painting technique. When a medium is mixed with
oil paint, the value can change. I noticed this effect especially for light
valued mixtures.. For some mediums, the value darkens again when the
paint dries. (ie liquin). Probably due to the evaporation of solvents? (As
the darkening happens for mineral spirits, but not for stand oil).
See also this post on www.wetcanvas.com: