I've been reading the book, "Vision and Art: the biology of seeing", and it's very interesting. The other day I had an "aha!" moment. It explained why one particular painting of mine looks very different in different lights. I noticed that depending on where I hung the painting, I would see more or less blue in the trunk of the main tree. And now, due to my reading, I know this is due to the Purkinje shift. It means that in low light levels, the relative brightness of blue and red can be reversed, because your rods, the cells that are responsible for night vision, are more sensitive to the blue end of the spectrum.