Why is a fire engine red?
Easier to see.
Not all are red, some places they are white, others places they are yellow. Just like not all police cars are blue. It depends on what color the city (or county) wants them.
Color of fire itself is what originally caused all items related to fire fighting to be RED.
The yellow mentioned above is probably "lime-green". The federal government did a study once and determined that "scientifically speaking" the lime green color was easier for the human eye to perceive than red. It recommended changing the color of fire trucks to lime-green.
The NYC fire department experimented with a few lime-green fire trucks to see if it would improve emergency response. After a year or so they concluded that it was actually worse than the red trucks because the public did not recognize the vehicle as it approached as an emergency vehicle because they were taught since childhood that fire trucks are RED.
Therefore, fire trucks continue to be RED because it would be too difficult to re-train the public to recognize a different color. Although the Green Goddesses (the fire trucks that are used when the fire department is on strike or otherwise incapacitated) are coloured a dark green.
Many departments have added WHITE to their RED fire trucks because it is more visible at night and the cost of the paint job is less.