While trying to fix my sand sculpture problem (different thread) I did some research into why things change colour when they are wet and don't always change back. I got into this because I didn't understand why grains of sand set in a completely transparent resin appear to be much darker than the pile of sand they come from.

I know that colouring of all the materials we use on this forum is of interest to some people so I thought I'd post the link below which I found helpful...although I had to read it about five times before I found it helpful. And actually it wasn't helpful at all in the sense that it helped to solve my problem, I suppose it was just interesting.

http://books.google.com/books?id=DNMIFlzVbk0C&pg=PA113&lpg=PA113&dq=%22clouds+in+a+glass+of+beer%22+%22multiple+scattering+at+the+beach%22&source=web&ots=8Pm7ZlgFnN&sig=JwFhG9KUm21_34_M1-RRYw89nDg