Night & Night
Even though Popeye and Cloudy are on the job 24/7, everything that goes down seems to do so under cover of night. Or maybe that's they way all criminals do their thing? (We wouldn't know. Promise.) Characters are constantly clothed in shadow or entirely silhouetted. The shots become high-contrast, slices of light and deep folds of dark.
Even though, for instance, Popeye, Cloudy, and the full squad bust Sal et al. during the day, it's a cloudy winter day without much direct light. There's even more darkness once Popeye ducks into the abandoned building after Charnier and the murder goes down. (Bye, bye Mulderig.) Perhaps it's that darkness that allows Popeye to shoot first, and ignore Cloudy later. Or would he have done it on high noon under a million-watt spotlight?
Similarly, in the very first scene, the French detective follows Nicoli (to his own eventual demise), it's in broad daylight—however, the windy, slim little alleyways of Marseille are cloaked in shadow. And when the French detective is shot, it's done out of the sunlight and in the hazy, muddy volume of an apartment building lobby.
Sure, it would be easy to think about crime as darkness, and justice as light, but this movie seems to refuse such a pat interpretation. The sunniest moments happen when Charnier is swanning about in Marseille, giggling with his wife or shaking hands with Devereaux.
What do you make of such an inconvenient break in the light = good, darkness = bad archetype?