The very earliest episodes featured Kirk in a yellowish-beige uniform shirt (Spock wore one too at times), but it got badly washed out under the lights.
Costume designers created a new uniform shirt of a brilliant lime green to set him apart from the red of engineering & security, and the blue of sciences, but to everyone's surprise the combination of the bright lights on the set, the non-reflective velour, and the film stock they were using made the shirt look more like the now familiar "command gold" than green.
Other uniforms made of different materials, including the casual wrap-around tunic he wears in certain episodes, and his dress uniform, do look more greenish.
It was also created to hide Shatner's growing gut since it wouldn't look right for a high ranking military officer in the "field" to have a big gut. See Starfleet uniform (mid 23rd century-2265) at Memory Alpha for more details.