A full set of uniforms (including Dress uniforms) were designed by the film's Costumer Designer, Bob Fletcher.
This was discussed at some length in the book "The Making of Star Trek : The Motion Picture" by Gene Roddenberry:
Fletcher not only devised uniforms for all the crew, but, as with most
military services, there are several classes of uniforms.
Dress uniforms are worn only on formal occassions and at Starfleet
Headquarters. The best example of this can be seen in Admiral Kirk’s
uniform worn at the beginning of the movie, with Bill Shatner wearing
the crisp loden green and white uniform.
