Gobs of merchandising color corrected the shirts in the 70s and early 80s. It was usually done to add eye catching color. Sometimes it was cost cutting/ effective. I had paper birthday plates when the animated series was on depicting most if not all characters in red shirts.
I seem to recall Star Trek The Phenomenon
being the name of a TV special from around the time The Motion Picture
came out. The host talked about the enduring appeal of the show and interviewed Leonard Nimoy. Perhaps, it's a flyer for that.