[This entry is under construction... fittingly enough!]
During the second movie, the four Ghostbusters occasionally wear hard hats either as a disguise or as a necessary tool when exploring underground. The shooting script called for Venkman to first wear a blue Con Edison hard hat when talking to the police, swapping it when the actual Con Ed supervisor shows up for a Nynex telephone company hard hat. The Nynex hard hat was illustrated as white, but with the incorrect name NYEX, in the NOW Comics adaptation.
In the actual film, the Ghostbusters only ever wear yellow hard hats, although some of them were noticeably molded in other colors and painted yellow for visual consistency. Venkman's hard hat sports a 'No Bozos' sticker above his right ear and a '100%' label at the back. Spengler's hard hat in the 'digging a big hole in First Avenue' scene has a sticker in the front which careful video analysis shows to be from a piece of airport equipment called a Runway Visual Range Remote Display Programmer. Two other small stickers above his ears are also present but have not yet been deciphered.
In this same scene, Stantz wears a stickerless hard hat with a battery-operated head lamp attached. He wears the same (or very similar) hard hat in the later 'tunnel' scene, where Spengler and Zeddemore also sport such lamped hard hats.
The production used several different brands of hard hats for these scenes, not all of which were yellow originally. Careful viewing will reveal continuity errors of the style of hard hat changing within a scene. As many as seven different hard hats can be seen throughout the film.
At least two of the lamped hard hats have been displayed among original movie props, with round 'Ghostbusters 2' stickers applied sometime after filming.
GBFan David (Ramcuda) has identified three of the brands of hard hats used. The only one that Venkman wears is an ERB style 906. The one first worn by Spengler is a Jackson Sentry Safety. And the ones worn by Stantz and Spengler in some scenes are Eastern Safety Tuf-E's with modified JustRite head lanterns screwed onto the front. These two hard hats are visibly painted yellow with exposed white plastic. The JustRite for Stantz has a red plastic battery case.