I've never heard of a colour film being considered a film noir but Niagara certainly has some of the other elements: murder, deception, infidelity, atmospheric photography in the tower, and a gorgeous femme fatale. The next time I watch this film I'll set my TV to black and white to see how it plays.
First of all, even though chiaroscuro lighting is a common technique in film-noir, it is not necessary even during the classic period. Keep in mind that many prominent noir DP's were from Europe (refugees fleeing Hitler) and were trained in German Expressionism. Furthermore, the vast catalog of classic noirs were mostly B pictures. Using the chiaroscuro technique helps to hide cheap sets.
What distinguishes "Niagara" as legitimate film-noir is that the Cutlers are used as a metaphor representing the well-adjusted, happy and stable mid-century couple whereas the fireworks between Marilyn and Cotten are firmly planted in the world of noir. Marilyn is the breathtakingly beautiful and sexy femme-fatale as she works her charms on her young, secret lover to commit murder. This is a classic film-noir plot.
To answer the OP's question however, "Niagara" was filmed in gorgeous three-strip Technicolor cinematography. Watching a BD on a properly calibrated display device, the picture has a warm, organic quality that is simply a pleasure to watch. The color reproduction is eye-popping with creamy highlights and excellent clarity. The observant viewer can even see the faintest peach-fuzz hairs on Marilyn's arms. This is candy-colored film-noir, the likes of which Hollywood will never produce again.
I've seen Niagara multiple times over the years and despite repeated viewings I find myself perpetually hypnotized by its velvety, sultry, saturated cinematography. I came to this board specifically to see if anyone else responded to the visual splendor of this film. Eureka! It was absolutely exhilarating to read your beautifully articulated response. And as long as I'm here, let me add that I truly appreciate your taking the time to express yourself on a forum that too often seems mired in negativity.
Your referencing Marilyn's peach fuzz reminded me of something I read long ago. Though I can't recall the source, I read that it was precisely this downy covering that gave Monroe the unique lushness that came through so magically on film.
I totally agree. I rated it at least 1 star higher than I would have on account of the cinematography alone. The use of chiaroscuro, shadows & silhouettes etc. is most unusual in Hollywood colour films of the period. The menacing backgrounds and the bell-tower staircase scenes are almost expressionist. The contrast with the post-card beauty of the falls is most effective.
==== Tony Why, oh why, didn't I take the blue pill?