As a dry summer passes into history, wet weather returns. And so do some of the colors we've been missing. Not only leaves, but rocks, soil, and road surfaces all have more intense color when they're wet than when they're dry.
Take a wet reddish-colored rock, for example. A wet rock is coated with a film of water. The water is smooth on the surface and it fills all the tiny valleys and pits in the surface of the rock.
When light from the sky strikes this wet reddish-colored rock, the water bends the light downward, into those tiny valleys and pits. The light gets reflected back and forth from one side of each little valley to the other. At each bounce, more non-reddish-colored light is absorbed by the rock surface. The light that bounces becomes fainter and redder. A small amount of light bounces back up to the surface of the water film, and some of that light passes through the water surface into the air on its way to your eye. So when you look at a wet rock you're seeing light that has bounced off the rock surface many times. A wet rock looks dark and strongly colored.
Now the case of a dry rock. There's no water to bend light downward into the little valleys and pits. When you look at a dry rock, you're seeing light that has bounced off the rock surface only once, or just a few times. Relatively little light is absorbed by the rock surface, so the light that reaches your eye from a dry rock is whiter and more intense than light from a wet rock.
Because water fills the nooks and crannies in the surface, not only rocks, but pavement, sand, soil, dead leaves, and bare wood look darker and more strongly colored when they're wet.