You know the ones - the little yellow ones.

I just noticed yesterday that they change colour between the first and second shots of the original Ecto-1 in GB2, going from a dark orange to the usual yellow.

What happened there?!

The surrounding environment changed.

If you look closely at the first shot, the white-walls, chrome, and reflections on the side windows of the Ecto-1 are all orange-ish-beige in color. This is either light directly from the setting sun, or sunlight reflecting off of the buildings behind the camera truck.

The orange-ish light plus evening shadows effects the appearance of the car's colors (especially warm colors like yellow and red).

In the next scene the orange hues are still their, but they don't appear to be nearly as strong. This shot is from a different angle, at a different location (even if it's the same street the car is moving, hence changing location), at probably a slightly different time of day.

Cosmic-Riptide;133425
The surrounding environment changed.

If you look closely at the first shot, the white-walls, chrome, and reflections on the side windows of the Ecto-1 are all orange-ish-beige in color. This is either light directly from the setting sun, or sunlight reflecting off of the buildings behind the camera truck.

The orange-ish light plus evening shadows effects the appearance of the car's colors (especially warm colors like yellow and red).

In the next scene the orange hues are still their, but they don't appear to be nearly as strong. This shot is from a different angle, at a different location (even if it's the same street the car is moving, hence changing location), at probably a slightly different time of day.

I thought that, but it didn't seem to affect the reds of the tail fins/n0-ghost logo or the orange of the ectospope thingy on the roof. i'm sure that is it, mind, i just liked the sense of ‘false ecto intrigue’!