Re: Why do objects sometimes darken when they are wet?

How light or dark an object looks depends on how much light is scattered 
from the object back to your eye. If a lot of light is scattered back, the 
object looks bright; if only a little bit of light is scattered back, the 
object looks dull or dark. Water often acts as a sort of anti-reflective 
coating, so that an object that looks quite light or bright will look 
duller or darker when covered with water.

If you want to know the technicalities, the amount of light reflected or 
scattered from a surface depends (usually, and roughly) on the square of 
the difference in refractive index between the materials that form the 
surface. Air has a refractive index close to 1, water is 1.33, and most 
solids between about 1.5 and 2.5. Because the air/water interface is smooth 
rather than rough, there will only be direct reflection from that surface, 
not scattering. The water/solid interface is the one that matters.

solid 				air R.I.			water R.I.			brightness
refractive index	difference squared	difference squared	factor

1.5					0.25				0.028				9
2.0					1					0.44				2.25
2.5					2.25				1.36				1.65

So a solid of low refractive index will look about 9 times brighter in air 
than in water, while a solid of high refractive index will look about one 
and a half times brighter.

You might also like to check out an answer I previously provided about <
A:href="http://www.madsci.org/posts/archives/may98/893874671.Ch.r.html"> 
Marmite