Filmmakers have to take certain liberties with the characters they adapt for the big screen, and one of the biggest changes they always seem to make is to the character's appearance, namely their costume and accessories.
There's something about the way those super folks look in the comics that filmmakers don't see as modern enough (except for Spider-Man's costume) which, in the case of Tony Stark's facial accessory, is understandable.
But why can't Black Widow wear a mask or Hawkeye a cowl or hood? And what's wrong with the bright color schemes Thor and The Hulk are seen sporting in the comics? Why can't Hulk wear his purple pants in peace?