How to add muddiness to a car after driving on mud?

I have tought about how to make this for a while now and cant figure out really any way. I guess it would be using same type of scripts like bullet holes and blood. I dont really know where to start off as there are really no posts of people making this before. (Yes you could say im new to scripting. I know the basics but thats it almost) Im still confused about how to make the mud texture to stay on the car. I know how to make it detect that the car is driving on mud or grass but im still confused about how to make the mud texture appear on the car? Should i use a texture on the car body that gets less and less transparrent the more you drive over the mud or should i use decals that stick onto the car somehow?