Films
The four films, and switching film any time during the day on the same 27 shots.
Since build 8
Every shot is taken on a film: Sunday (warm golden prints, the default), Linen (soft and creamy, kind to faces), Afterparty (flash and glow) or Silver (black and white). The film's look goes on the photo or clip before it uploads, and it stays with that shot.
Switching film
Tap the canister at the top right of the Camera to open the film sheet, and pick a film. The next shot uses it. You can switch as often as you like during the day: a shot taken on Silver stays black and white after you move on to Sunday.
Switching doesn't change the count. The 27 shots a day are for every film together, and the roll still starts over at 4 AM. On a shared roll the same is true for that roll's own count.
Durpin remembers the film you picked last and starts on it next time.
Before build 8 the film was chosen before the first shot and locked for the rest of the day.
Photos from your library
Since build 18 each photo you add from your library gets its own film, picked on Frame it with the photo shown on it, so you can see Silver or Afterparty on that photo before it goes on the roll. It starts on the Camera's film; picking another doesn't change the Camera's. Since build 19 a clip from your library gets its film the same way, on its own screen, shown on a still from the clip; the next photo or clip starts on the film picked last. More in The Darkroom.