The most widely accepted reason that fire engines are painted red dates back to the 1800s - a time when there was a lot of competition between the fire brigades of neighboring cities and towns. The firefighters of each brigade took great pride in their pump. Brigades wanted their rig to stand out by being the cleanest, having the most brass, or being a regal color. Because red was the most expensive color, that is what color most crews chose to paint the pump.
Other sources cite the tradition of painting the fire engines red going back to the early 1920s. Henry Ford wanted to make cars as inexpensively as possible and only offered cars in one color: Black. With all of these black vehicles on the road, the fire service began painting their vehicles red in an effort to stand out.