Assuming multicolored tractors with uniform overcast lighting, it seems similar to the case of shooting sports, particularly indoors.
In shooting basketball, with light and dark toned jerseys, you will drive yourself crazy trying to use autoexposure. The light is not changing, but the camera sees different tones based on what you are focusing on and tries to compensate.
Your best choice with constant lighting is to take test shots to establish exposure, set on manual, and the click away.