When WOK went into production, Paramount set a very strict budget for the film in order to avoid the huge over-runs incurred by The Motion Picture (TMP). To that end, anything that could be re-used was pulled out of storage, including sets, models, and the hundreds of costumes from TMP.
But director Nicholas Meyer wanted his Star Trek to look nothing like TMP, which he felt did not embrace the adventure of the original Star Trek series. So while his budget was limited, Meyer wanted his music, costuming, set design, language, and themes to be as far away from TMP as possible. And one of the things he hated most were the drab, boring uniforms used in TMP. They were made to be very utilitarian but they weren't terribly exciting.
To that end, he asked costume designer Robert Fletcher to do the impossible – give him new uniforms that looked nothing like the old ones, without having to create everything from scratch. Fletcher rose to the occasion and gave the main characters – Kirk, Spock and their fellow bridge officers – the new Class A maroon uniforms, while every background crew member would be wearing recycled jumpsuits originally made for TMP, but with a twist.
It wasn't as simple as just dying the TMP jumpsuits red, though. Fletcher also developed a cream-colored shoulder piece and various other stitched details that, when added, removed any sense of the original TMP design and changed them into customized companions of the new Class-A officer's uniforms.
I have owned four different examples of the jumpsuit and have noticed that there were definitely variations in the maroon color from one example to the next. This could be due to a couple of factors. First, while I think they started with the white TMP jumpsuits, I doubt that they had enough of those and so had to use either tan or gray as well which would yield some variation. Also, since the pieces were dyed individually (rather than their cloth being dyed before construction) variations in color could have entered in based on time in the dyes. Whatever the reason, the variations exist, but they had no impact on the filming. Under the bright lights of the sets, one maroon jumpsuit looked much like the next.
So a design driven by the necessity of film budgets became a very successful component of the new Starfleet look brought about by Nick Meyers and Robert Fletcher. The transformation of the TMP jumpsuits was so complete as to be undetectable, while being so well done as to be indispensable. These then became the standing crewman uniform for an entire decade of Star Trek films.
Thanks for reading.