When the executives at Control Data Corporation found out that Universal was planning a major movie featuring a computer, they saw their chance for some public exposure, and they agreed to supply, free of charge, $4.8 million worth of computer equipment and the technicians to oversee its use. Each piece of equipment carried the CDC name in a prominent location. Since they were using real computers - not just big boxes with a lot of flashing lights - the sound stage underwent extensive modifications: seven gas heaters and five specially-constructed dehumidifiers kept any dampness away from the computers, a climate control system maintained the air around the computers at an even temperature, and the equipment was covered up at all times except when actually on camera. Brink's guards were always present on the set, even at night. The studio technicians were not allowed to smoke or drink coffee anywhere near the computers.
Universal later reused the footage of Colussus being activated as part of Cyborg: The Six Million Dollar Man. Before they begin operating on Steve Austin to attach his bionic limbs, the entire activation sequence from Colossus is used.
In the movie, one of the attempts by the humans to regain control of Colossus is to try to overload the machine by feeding it too much data. This sequence is not in the original D.F. Jones novel "Colossus", on which the movie is based; however, it is a major plot point in the novel's sequel, "The Fall of Colossus", which was published in 1974.
Producer Stanley Chase said that while it's frightening to suppose a computer could take over the world, it was indeed possible. His technical advisor said that a machine like Colossus actually existed at that time. The model for Colossus was supposedly the NORAD system that controlled the US national defense systems, and that's why the computer programming center in the film was located in the Rocky Mountains (which is also the home of NORAD). The US government wouldn't allow a film crew on the NORAD grounds, so the exteriors were filmed at the Lawrence Hall of Science in Berkeley, California. The missile sites were photographed in the California desert near Palmdale.
When the Colossus commanded missile re-targeting procedure is shown in a segment where the conspirators are watching for the success their sabotaged warhead arming module replacement, the new target coordinate provided by Colossus (Lon 99 deg 6 min 45 sec, Lat 19 deg 26 min 5 sec) is that of Mexico City, Mexico.
Programs and commands for Colossus often elicit the expression "OLD PROGRAM NAME" on the large text displays of the computer. This reflects the influence of time-sharing terminal interfaces used at the time, especially for interactive programming environments in the BASIC language. Existing programs were often loaded for editing or execution by typing the command OLD into a terminal, and then responding to a prompt with the program name, whereas new programs were created by typing NEW. Similarly, RUN was the command used to actually execute a program written in BASIC on these systems.
The outside views of the "Colossus Control Center" are images of the Lawrence Hall of Science, just opened in 1968 when this film was being made. LHS is a science and computer museum still open to the public in the hills just above the University of California Berkeley campus and is a University managed facility.
The trivia item below may give away important plot points.
When Colossus/Guardian detonates a nuclear bomb near the end of the film, the blast footage seen on the monitors and TVs is that of "Ivy Mike", the first US test of a nuclear fusion device. It took place in November 1952 at Enewetak Atoll in the Pacific Ocean.