Kingpin
The hazard stripes above the doors on the actual car were orange. The only Tonka yellow ones on the film car were the ones running along the sides of the vehicle at logo level.
Actually KP, I don't think that's correct. The caution stripes were a reflective yellow/amber and black striped tape. The nature of the material makes the stripes above the door appear orange because of their angle in relation to the viewer and surrounding light sources.
The same thing happens when you pass a traffic sign (as you move from in front of the sign to it's side the bright white lettering turns a smokey gray).
You might have to copy and paste the following links to make them work…
http://www.jackal86.100megs8.com/mastertemp/Ghostbusters-Rel/E1A1.jpg
http://www.jackal86.100megs8.com/mastertemp/Ghostbusters-Rel/E1A2.jpg
It's not the best reference for my point, but notice in the second image… the tape that runs along the center of the doors… towards the back where the tape turns and connects with the car's fins… as the surface of the car bends the tape's yellow appears to turn the same shade of “orange” as the tape above the doors.
I believe the orange is an optical illusion created by the slope of the 1A's roof and the properties of the tape.
Note the photo of the bike halfway down this page…
http://www.mechbgon.com/visibility/passive.html
The yellow turns to orange as it curves away from the viewer.