The paintable sludge as used in the HoF 2020 can be built into your own map. Here are the files for installing the sludge as used at HoF 2020.
The mud can be painted on with the ingame tool.
For this I also recommend incorporating the texture lettering so that this is better visible.
The mud is only displayed under these conditions. without seasons: at least 5% soil moisture with seasons: at least 20% soil moisture and no minus temperatures.