Why are the all stills and the trailer for this film in B+W?

I'm afraid I have to disagree, I've seen many, many, many stills for this movie and more of them are in colour than in black and white.
There are 2 trailers for Niagara, the main one is colour, the other is black and white.
They did the same with another Marilyn film released the same year 'How To Marry A Millionaire', it has 2 trailers, one in black and white, the other in colour, I don't understand why they'd bother making a black and white trailer for this film as it was the first *but second released* movie made in widescreen, so you would think they would want to advertise it with a colour trailer.
Anyhoo.. here's the colour Niagara trailer:

http://www.youtube.com/watch?v=sYRNS110lwg

The black and white trailer was made for television which was not in color in 1953. Many of these still survive and sometimes outsurvive the color ones for movie theaters because more copies were probably made like one for every tv station in a city vs. just one color one for the moviehouse showing the film.

The stills are in black-and-white because all movie stills released by the movie studios (with rare exceptions) back pre-1960 were in B&W, even for Technicolor musicals. This was because they were taken for use in newspapers and magazines that almost printed in color unless it was an exclusive shot for the publication.

There were color photographs taken at the time but they were not used as "stills" for conventional publicity markets but perhaps for magazine covers or major pictorials in national magazines. Color film was dramatically more expensive than B&W back then. If you collect memorabilia you'll note Fox often released "colorized" B&W stills for use in movie theater lobbies rather than actual color photographs because the process was much cheaper. I have a number of hand-colored stills from several Marilyn and Jayne Mansfield films released by Fox 1953-1957.