Color, like sound, widescreen, and digital cameras, is a special effect that was implemented as a form of spectacle/innovation and ended up becoming a standard amongst filmmakers in the industry. It’s rare that a film is not in color nowadays...
The MPAA (Motion Picture Association of America) seems to show a particular bias towards sex rather than violence, especially when considering the "R" and "NC-17" ratings for a film. The smallest sexual reference can skyrocket ratings even over the goriest...