S.I.G. is an initialism used in the television series Captain Scarlet and The Mysterons, it stands for "Spectrum Is Green", and has a similar use to "10-4" or "Roger".
An alternate is also used: S.I.R., which stands for "Spectrum Is Red"; it means "negative".
It can be likened to the term "F.A.B." in Thunderbirds and "10-10" in Terrahawks.