Also, I remember reading that when the new producers were planning the look of STII, they made a deliberate decision to go with "almost" but not quite primary colors -- hence maroon instead of red, teal instead of blue, etc.
I own that costume in the middle. Tellarite ambassadorial robes by Therin of Andor, on Flickr Tellarite robes by Therin of Andor, on Flickr
Keep in mind also - wardrobe has to be washed every night after shooting. I'm sure that contributed to at least some of the color variation with some of these outfits. Though I tend to agree - I never interpreted the engineer stripes as purple or magenta; I just assumed (based on the yellow and blue and red template previously established by the other series) that it was red.
The same costume guy, Robert Fletcher of TMP, did the later movies too. He hated "straight" colours and used the same strategy in that first movie. Spock's black robe wasn't a true black, Kirk's admiral uniform was "loden green", but appeared grey to some, etc.
^^ Fletcher, yes. Thank you. I liked his designs -- really nice lines. I also liked the move away from the "straight" bright primary colors of TOS to the more muted, off-primary colors. The seamstress in me appreciated the variety, I guess.