On tire materials there are 3 float 0-1 parameters that were intended to be changed dynamically,

• _FlatTire
  
• _Mudness
  
• _Wetness
  

On wheel material there is similar float parameter _Mudness and _ColorA, _ColorB and _ColorC.

Where RGB values are just coloring based od Blending texture in Blue of MaskMap texture and in Alpha of _ColorX is Smoothness control.

On materials there is slot for tileable mud/snow texture, there are included two Mud_DiffuseSmoothness tileable textures: MudDirt_BcR and SnowDirt_BcR that share normalmap. By default materials are setup with MudDirt. But if You intend to use it on snow or color match to ground color, You could use SnowDirt texture and change _Mud_Color parameter.