Making mud with putty?

You could do it undoubtedly, but it would be expensive and the putty may not be friendly to some bases, eg. if you were to use styrofoam as a base, the putty would dissolve or soften it. It might also be difficult to work with, given that it dries out (relatively) quickly.

A better, cheaper option would be ready-mixed vinyl spackle from the hardware store.

EDIT: On re-reading your post, I now realise that you were talking about mud on vehicles. When I wrote the above, I thought you were talking about mud on a diorama base....

I do it all the time, although I prefer using cheap Testor's putty on the hulls, roadwheels, and/or tires..  Thins with rubbing alcohol and applied in a stabbing, stippling action with a short-bristled brush...  

Mud's color depends on the track's AO, from the dark gray dirt of the Central US to Euro-Brown, to the red clays of Georgia, Oklahoma, and the Central Highlands of South Vietnam ... 

 I wouldn't use expensive model paints with FS numbers or even more expensive pigments though.. Just go get some tempera paints (I keep the primary colors plus black & white, on-hand), and use those instead...  Craft paints are cheap and have plenty of "earth-tones" too...

With Red, Blue, and Yellow tempera (either powder or pre-mixed), along with the black and white, you can make any shade of mud you can dream up...  They can also work well as mud themselves when mixed in with talcum powder...

Sheet-rock mud works well too... As does real mud...

you could mix the putty with the paint or even weather pigments or pastels however it will likely effect the consistency and time of the putty to dry. experiement and see what you may get. if all else fails just go get some dirt and m=use real mud it's certainly cheaper!

OK, thanks for your tips. I'm building a braille-scale kit so I won't waste too much putty. I have water-based paints, so I'll try with them . Best regards.

José.

Wow.. In that scale, most of the "mud" would be 2-D, i.e., paint..