The face and voice of Captain Scarlet were both based on Cary Grant. In fact, Captain Scarlet's voice artist, Francis Matthews was chosen to voice the character based on the fact he could do a Cary Grant impression. In fact series creator Gerry Anderson came close to moving heaven and earth to get Matthews who had been either unintestered or unavailable. Anderson managed to convince Matthews to divide his time between onstage appearances in Noel Coward's play Private Lives and the Captain Scarlet voice recording sessions.
The voice of the Mysterons, and hence that of Captain Black as a Mysteron agent, was produced by having Donald Gray read his lines as its provider at normal speed whilst the tape recorder was accelerated; when the tape recording was played back at normal speed, the sound of Gray's voice became deeper and slower than his own, and hence more ominous and menacing.
Real human hair was used for the marionette heads. Photographs of actual human eyes were used to increase the realism of the marionettes. After producing several series using cartoon-like marionettes with large heads (required to contain the electronics that operate the puppets' eyes and mouth mechanisms), Gerry Anderson was able to miniaturize the mechanisms further starting with this series, allowing more realistic-looking puppets to be used.
The Spectrum organizations' radio code SIG stood for Spectrum Is Green (essentially an OK), whilst the less used SIR stood for Spectrum Is Red (indicating a dangerous situation). Both radio codes have been borrowed by American private spaceflight firm SpaceX to describe its readiness for rocket launches, modified to "SpaceX Is Green" or "SpaceX Is Red."
According to Gerry Anderson on the documentary for the 2001 DVD release, The Mysterons were written as an invisible enemy in case it was proven that there was no life on Mars, he could say there is that you just can't see it.