Controls and various other items are required to be labeled. Some have the potential for serious consequences if activated at the wrong time. Imagine a non-pilot passenger in the rear seat of this RV-8 pulling the throttle back at takeoff, thinking that the black lever must be for fresh air! Fuel caps are required to be labeled with quantity and type of fuel. There have been way too many accidents with aircraft fueled with the improper grade of fuel.
Labeling the fuel filler cap areas along with all control levers inside and outside the aircraft will educate the passengers. However, a passenger briefing is the other half of the education needed.
Absolutely agree and it is required. Having given lots of rides to first-time passengers, many times thier anxiety causes them to not absorb a whole lot of the briefing.