I'm sure someone can confirm this, but I seem to recall that Stephen Nichols had been cast not long at all before this picture was taken, and his own uniforms had not been made yet, so they found him one that would fit - and it was a "gold" one-piece. (I don't have the picture in front of me, but if memory serves, this is one of the photos taken before Spock's undershirt/collar was made, as well.)
I can't confirm any of that, but there's some publicity photos that would seem to jibe with what you're saying. Here's a more "candid" group photo that was likely taken when they took the official group photo — several of the cast members look like they're laughing, and GR and Bob Wise are in the photo as well.
In addition to Stephen Collins in the gold jumpsuit and Nimoy without his collar, Shatner's wearing an interesting uniform variation — a two-piece with shoulder tabs. IIRC, it was only the jumpsuits and the short-sleeved uniforms that had shoulder tabs.
Kirk/Spock/McCoy group shot. Again, no collar for Nimoy, and Shatner's got shoulder tabs. Maybe it's just the lighting or the film they shot this one on, but the uniform colors are really
Group shot of just the TOS cast members:
The uncropped version of the official cast shot — notice the wooden beam hanging from the ceiling.