* I thought this has been asked before but a search revealed no results*
Your assumption was correct, your search function was not. Hay, that sort of rhymes!
On TNS radio show of the 18th Feb 2007, a similar question was asked and this is the reply given:
"When hair gets wet it gets closer together. Hair is light when it's dry for the same reason that glass is clear when it's as a glass but lighter when it's made into tiny particles of sand. What's happening when it's dry is that light is bouncing off all the surfaces of the hair and bouncing around all over the place and reflects lots of wavelengths of light and looks lighter. When the hairs are covered with water it smooths the surfaces, more light can go through deep into the hair rather than be reflected back, so it looks darker."
Which is ofcourse, what i was going to type, but without the big words.
As a side note, copper turns blonde hair green. So maybe you could "alter" his diet for some family green haired fun.