Children get to an age when everything is questions, questions, questions.  My daughter, who is now 36 remembers all of my answers to her questions a third of a century ago.  She just recited back to me, why a fire engine is always red:

"It is very simple and very logical.  Fire engines carry hoses in nominal lengths of 100 feet. One foot is 12 inches which is a ruler.  England's ruler is Queen Elizabeth.  Queen Elizabeth is a ship.  Ships sail on the oceans.  The oceans have fish.  The fish have fins.  The Fins fought the Russians.  The Russians are Reds. And THAT is why fire engines are red, it is because they are always rushin'."

Now, at 36, she asks, "What about the pukey yellow-green ones, Dad?"  Any takers?  Methuselah??  Help!!

cuz it was cheaper to get it in that color??

National Highway Traffic Safety Administration (NHTSA) determined that yellow-green fire trucks with flashing red lights were seen more easily in evening light by motorists than the darker red fire trucks with flashing red lights. This must have been an earth shattering awareness, as NHTSA normally can't tell a ferry from a horn up a bull's ass.  Where was this concept generated?  Does Red Green have a blonde brother, Yeller?  Now, that would make logical sense.  I can see their Canadian inventiveness at work in such a postulation.... RED??

To hold their pants up!

Reminds me of the little girl on the old Fibber McGee and Molly radio shows on XM.

Actually the reason is they are easier to see at night. At least that's what I've been told.