Exoray’s kit certainly assigned a red LED to the rear of the two lights on the top of the Proton Gun, combining the best of the “hero” prop and the “background” props.
Here’s some observations of the gun lights from Ghostbusters:
The following setup is seen during the majority of the film, the most detailed observation being from the Sedgewick scenes:
Slowblo/rear instrument bar: lit, static. (Sedgewick Hotel bust)
Vent: lit (very bright), static. (Sedgewick Hotel bust)
Bargraph: Fully lit, static. (Sedgewick Hotel bust)
Forward top indicator light: lit, flashes steadily on and off. (Sedgewick Hotel bust)
Rear top indicator light: unlit. (Sedgewick Hotel bust)
White hatlight: unlit. (Sedgewick Hotel bust)
Orange hatlight: lit when proton stream is fired (and afterward as the Ghostbusters investigate the full Trap). (Sedgewick Hotel bust)
Barrel light: Random white flasher. (Sedgewick Hotel bust)
Switch note: Suggested in the Sedgewick scene and finalised during the Gozer bust, the lower DPDT switch activates the vent light. The upper switch on the Gozer battle Hero Pack is also activated, potentially supplying the power to either the hat light or the bargraph.
The following setup is only seen once, on the hero pack seen only during the battle with Gozer:
Slowblo/rear instrument bar: lit, static.
Vent: lit (bright), static.
Bargraph: Fully lit, scolls up and down.
Forward top indicator light: unlit.
Rear top indicator light: unlit (features sunken orange hatlight).
White hatlight: lit, but very dim.
Orange hatlight: unknown.
Barrel light: unknown.
Although it forgoes the front white hatlight, Jupiter’s kit is pretty accurate to the light setup seen through most of the film, and obviously gives the added bonus of an animated bargraph (just make sure to request a static vent light). As far as I can find through the shots in the film, the rear indicator light on the Proton Gun is never illuminated.