Fire Trucks are painted red to make them stand out and be seen.
Fire Trucks are red as to make the Fire Trucks stand out and be seen.
Because red was the most expensive color of paint, volunteers used red to make their fire trucks stand out as a source of pride.
Of course, these two theories are related but opposite.
Another theory holds that fire trucks were painted red to make them stand out from all the other vehicles on the road.
However not all Fire Trucks are red and in some places Fire Trucks are other colors including Yellow.
Hawaii is a place that has yellow fire trucks.
Sometime around the 70s a study decided that yellow was the most visible (and therefore safest) color at day and at night for fire Apparatus.
Before that Honolulu Fire Department trucks were red, but switched to yellow in the 80s, and they've simply never changed back.
Fire Trucks are also sometimes Blue and other colors but Red is the most common color of fire trucks.