Sample the pants colour, and add a Solid Color layer of that colour. Set that layer's blend mode to "Color". Mask it to the wet patch plus some of the surrounding pants so the colour blends.
Carefully select the wet patch, then add a Levels layer BELOW the Solid Color layer. Adjust the middle slider until it matches as closely as possible.
At this point you might want to go back to the Solid Color layer and tweak its colour a bit.
Once you're as happy as possible with all that, add a new blank layer above the other layers, and clone on it. No matter how careful your selection was for your Levels layer, there will still be edges visible. Gently clone over those.