I hear some people say that red paint was the cheapest color since all barns were painted red. Others say it was the most expensive color hence why Ford painted the all model T's black.
The cost of the paint was not much of a factor. Fire trucks are painted red because of fire brigades dating back to the 1800s and early-mid 1900s, when there was competition to the act. Fire trucks, now, are painted red for visibility - both because they are emergency vehicles, and because Henry Ford painted his model T's black, so the fire brigades painted them red, for visibility.