For the benefit of those outside the US, most fire departments are handled at the local level, so even a medium sized county might be served by seven or eight fire agencies -- one for each city, and at least one for unincorporated areas. Despite the decentralized nature of fire fighting, a fire truck in New York will look almost identical to a fire truck thousands of miles away in California. What makes this even more confusing is that other emergency services, like ambulances, do not have a single standardized look, which begs the question: why are almost all US fire trucks painted similarly?
This is actually surrounded in mystery, and goes back to when fire wagons were used, pulled, and pumped by hand. Some sources say its because almost all of those hand pumped fire companies were volunteer, and red paint was extremely cheap and easy to get hold of and thereby setting an unofficial precedent, some say its the exact opposite! Other publications say that because almost all of these fire companies operated independently, and the competition between them was so heated that they did there best to look as nice, and professional as possible, and back in the 1800's bright red underlined with gold looked best, setting the precedent. Theirs even a third theory that they were first painted red to stand out among other carriages. So there's three widely accepted theories, none of which make any less sense then the other, so its just a matter what you choose to believe. At this point fire trucks are painted red is mostly for tradition. (there are other colors, but red happens to be the primary one in most cases.)
Although OP didn't specifically mention a color, I'd like to offer this side note: Fire trucks haven't always been red. I seem to recall that in the 80's (at least in my part of Canada), the traditional fire engine red was dropped in favor of bright lime green. I believe it was to improve visibility of the truck. But in more recent years, in nod to tradition, they have returned to fire engine red.
A side side note: I visited New York City about four years ago, and was delighted to hear that the fire trucks there were using the old traditional-sounding sirens with the slow rise and fall of pitch. I don't know if they have always stuck with that or if it was a nostalgic return to a classic.
Even as far away in China the fire trucks are painted red and look similar to fire trucks in America.
Source: I live in China
