At about the 3 minute mark, Tom and Diana are in a darkroom with a red safe light on, and a half minute or so later, Diana is taking color prints off a drying line. A red safe light light is only used for B&W darkrooms, since many B&W films and papers are only sensitive to blue and green, and relatively insensitive to red. A red safe light is never used for color, for obvious reasons - color film and paper is sensitive to, well, color (all colors).
When first looking at the photos Diana tells Tom to match the photos to the electoral roll so they know who the owner is. Later however they are surprised when Harvey implies one of the houses belongs to the perspective MP.