That's not actually true.
White is for all command department officers. Not just the actual ship commanders.
Even though some jobs that had been (and would again be) part of Command got shunted off to Operations or Sciences for some strange reason, like helm and navigation, that doesn't mean there weren't any lower-level Command officers.
I'll admit that the Monster Maroon color scheme doesn't actually make a ton of sense, though.
The folks in this picture may not all be CMDR's or Captains, but the few whose rank I can see clearly would be in specialized roles that would definitely mean that, even if you did not need a certain rank to wear white, any post outside of a Command Headquarters would likely have very few of them.
-Saavik is only a LT, but unless I'm mistaken, she and the Captain are the only 2 people in a white sweater shown on the Grissom, and she seems to have some kind of lead role.
-The bald alien works at HQ and also is giving important (weather) reports, so that indicates that he might be at least an important NCO in communications or meteorology or some other role like that.
-I cannot tell anything clearly for the two men in the upper small pictures, other than one is a LT at CONN or Ops. That person could be XO at the rank of LT and still do one of those positions, like Number One in "The Cage." What movies or episodes are these from?
-The woman in picture on the upper right is wearing an honor cord, and since Kirk's yeoman wore an honor cord in ST:5, she is probably a Captain's yeoman and is wearing white for that reason, so there would not many others wearing that uniform. Again, I'm not sure what movie this is from.
-The woman in relative close-up in the lower right is on a variant of the movie bridge, but it does not look like a variant used in the actual movies. I'm guessing that she is an officer shown in the background of the Bozeman, am I correct? She seems to have CMDR's or Captain's ranks, which would make her possibly the first officer of the Bozeman, which does not require than many of the Bozeman crew wear white.
It's admittedly hard to tell in these pictures since some details are not clear, and saying that only 3 people on the ship wear white may be a stretch, but it seems plausible that under this system we would generally see a lot fewer officers in white sweaters than we would see officers in red shirts in TNG.
It probably wasn't helped by the studio never releasing an official guide. Publicizing that information would have made it a lot easier for the production crew to follow it.
Maybe had they explained what the colors and symbols meant, later costumers could have avoided some of the more embarrassing mistakes.
On the special features for one of the movies' DVD's, Robert Fletcher alludes to creating the rank insignia for these uniforms and coming up with a system for departmental colors. Apparently it is either complicated, it was changed by later productions, or they just costumed actors as they could.
This example of Valeris' uniform colors does not bother me much; I just suppose that she is a recent graduate who got her shoulder strap at a ceremony, but who has not gotten her Sciences sweater yet
