There’s this Devo song called S.I.B. (Swelling Itching Brain), so my mind was playing around with S.I.G… swelling itching goiter? Swelling itching gallbladder? Swelling itching gerbils?
As I had come across Captain Scarlet before the MST3K episode I’ve already knew the SIG/SIR Spectrum is Green/Spectrum is Red before hand. It’s just one of those things I’ve taken for granted without even thinking others might not know the meaning.