You can see wet-dry maps in action in a downloadable scene: → "Splash to Wall Wetmaps".

RealFlow | Cinema 4D is able to read an object's UVW coordinates for applying textures or creating wet-dry maps. If the UVW coordinates are wrong or not present you will not get the expected results and the calculation of wet-dry maps might fail. Here is some more information on them:

You have to assign a → "Collider" tag to the object you want to make wet.
The tag provides the parameters to control the map's quality and drying effects.
RealFlow | Cinema 4D's wet maps are always squared.
Wet maps will be → cached together with the particle simulation. Then it is possible to post-process and reapply the maps.

Follow these steps:

Assign a "Collider" tag to an object, open RF Collider tag > WetMap, and make your → settings.

Make sure that the object has proper UVW coordinates.
Create a material and select the "Color" channel (or any other channel with image support, except "Normal").
From "Texture" choose "RealFlow WetMap".

Attach the material to the object.
To see the maps at simulation time, go to the material's "Editor" tab and enable "Animate Preview":

Improving Quality

The object's → "Cell Size" value determines the quality of the fluid-object interaction. If you can see gaps between particles and surface decrease this value:

By default, "Cell Size" is calculated automatically, based on your → "Collision Geometry Detail" selection.
When you disable the "Cell Size" parameter's "Auto" option you are able to enter a custom value.

It is possible to change the wet marks' colours:

Select the material and open RealFlow WetMap > WetMap Display.
Adjust the colour gradient.
Be aware that these colour changes are temporary only unless you → (re-)cache the simulation.

Results

Here you can see a screenshot from a simulation and a cached map:

Browser not supported