This fixes the color issue with your mod, however 6-wheeled vehicles are currently best set to simple mode, as the current version of Real Dirt Color's independent mode is meant for a maximum of 2 axle vehicles. Anything more becomes susceptible to using the body's dirt color and won't change like the rest. Your body was using the fs17SupportShader which again doesnt have any "dirtColor" parameters in the materials, so it sets them to a nil value which ends up as zeros (black).
So all you have to do is download that shader, open the tractor's i3d in notepad and change the path to the fs17shader file to the new one. This is a simple way of affecting all materials at once. Then you must open and save the file, by doing this GE will write the "dirtColor" parameters from the shader to each material automatically. You can see the difference in notepad before and after. Done..
The color is now proper but your front right / and rear left tires still use the body dirt color (like simple mode) and will look wrong when at full color and on a different dirt colored surface (which is why the current best option is to use simple mode, so they are all the same).
I apologize I can't get this one perfect atm.
independent mode (but all vehicle parts are on the same surface)