That was a different fabric that actually photographed as its true color

https://web.archive.org/web/20100917090118/http://www.startrekpropauthority.com/2008/05/bill-theiss-lost-interview-rare.html

Another quirk involving the original series’ tunics were the colors - in particular, “command.” Trekkies everywhere will swear Spock wore blue, Scotty wore red and Kirk wore gold. Wrong. The three Starfleet colors were blue, red and green. Lime green, to be exact. “It was one of those film stock things;” Theiss states, “it photographed one way - burnt orange or a gold. But in reality was another; the command shirts were definitely green.” As further proof, look at the wrap-around tunics as well as the dress uniform tunics of Kirk’s – all green. They came off as their true colors because they were constructed of different materials than the standard duty command shirts.