Final answer:
The tractor air supply valve is typically red to make it easily recognizable and distinguishable from other control switches or valves in the tractor.
Step-by-step explanation:
The tractor air supply valve is typically red. This valve is used to supply compressed air to various systems in a tractor, such as the braking system and air-powered accessories. The red color is chosen to make it easily recognizable and distinguishable from other control switches or valves in the tractor.