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.
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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.