I would use something simple that doesn't have to be at full opacity/interfere with the readability of the text.

If you are experience high pressure's in the cooling system, replace the thermostat and radiator cap, and bleed the system with the heater on when you are finished. Sometimes raising the front of the vehicle helps the little air bubbles float up into the radiator. You want absolutely all air out so that the cap can properly expel or intake coolant into the reservoir as necessary. When there is air in the system, the air absorbs the pressure and causes higher temperatures before the thermostat or radiator cap do their job.