My question is as simple as the title gets.
I haven't finished watching TOS yet, so if I'll notice such occurrence I'll make sure to update here.
TOS Episode, no.
In the 1971 annual, Kirk's uniform was red. No explanation was given for the colour change.
Not as any part of his regular duties. I am also almost certain that he never did as any sort of disguise.
Red shirts (uniforms) signified engineering & security; blue was science & medicine, and gold was command.
If the green variant was ever explained, I missed that one. But since we always saw Kirk in a command position, never as an engineer or science position, we always saw him wearing gold -- at least in the original TV series.