Terrain texture manipulation

Hi, I'm trying to figure out a way to darken the color of a terrain texture without creating multiple different variations of the texture,

Practical use: I have "Soil" that players can "Water". When this happens, I want the soil to darken. Generally I would swap out the terrain texture with a darker version of the same texture. But I'm trying to go for something different where the terrain area slowly lightens to the normal color as the water drys up.

I was thinking of maybe adding a plane over the top of the terrain that is a dark transparent, and it slowly removes alpha.. But perhaps there's another way?

Thanks.
-Eddy