A British ambassador called the film "an insult to the nation". The then Lord John Brabourne read an early draft and called it "the most evil and perverted script I've ever read. It must never see the light of day".
Contrary to the story that says some scenes of the film are in black-and-white instead of color because the production company was running short of money and saved money by having some scenes processed in monochrome, according to interviews with Malcolm McDowell, Lindsay Anderson and the cameraman, they first shot the scenes in the school chapel in monochrome because they had to use natural light that came in through the big stained-glass window, requiring high-speed film. The high-speed color stock they tested was very grainy and the constantly-shifting color values due to the angle of the light through the stained glass made it impossible to color-correct, as well. So they decided to shoot those scenes in monochrome, and, when he saw the dailies, Anderson liked the way that it "broke up the surface of the film", and decided to insert other monochrome scenes more or less at random, to help disorient the viewer as the film slipped from realism to fantasy.
Paramount hated the film when they saw it and tried to dump it from cinemas. However, one of their tentpole films, Barbarella (1968), turned out to be a spectacular flop so they needed to replace it in cinemas with something else. Reluctantly, they wheeled out "If..." and were astonished to see it turn into a big critical and commercial success.
Mick and the Girl rolling on the café floor naked and making love was Malcolm McDowell's idea (because he wanted to see his attractive co-star, Christine Noonan, for whom he admitted having a crush, in the nude.) However, when Lindsay Anderson accepted his star's suggestion, the director required McDowell to ask Noonan if she was willing to do so. (Her reply, according to McDowell, was "I don't mind.")