Operations (TOS: "Wolf in the Fold"):
Sciences (TOS: "Space Seed", "Star Trek II: The Wrath of Khan"):
Command (TOS: "The Immunity Syndrome"):
Sciences (TOS: "Where No Man Has Gone Before"):
Command (TOS: "Arena", "Star Trek VI: The Undiscovered Country"):
Operations (TOS: "Mirror, Mirror", "Star Trek II: The Wrath of Khan" publicity shot):...
I'm gonna go for the EMH. He appeared in both the Blue and Red uniforms in the main series and his diagnostic subroutine (arguably a part of the EMH package of programs) appeared in a yellow uniform in Voy: The Swarm.
As @sidney has pointed out, the Diagnostic Program was ultimately grafted onto the EMH prior to the other two photos below, meaning that all ...
According to Memory Alpha, it's a "skant":
Introduced alongside the standard duty uniform jumpsuit, a skirt or "skant"-style uniform was also available to Starfleet officers as early as 2364. Similar to its jumpsuit counterpart, the skant uniform was a short-sleeved one piece skirt, that could be worn with or without trousers, and included thigh high, or ...
Fairly simple: Worf is a respected officer with a spotless record, thus he gets a bit more tolerance in some areas. Ro was court-martialed and had to be sprung out of prison by an admiral, therefore Riker has her toe the line like a first-year cadet. She was out of uniform upon arrival, and got snapped at for it.
In reality, Worf was wearing the sash in ...
Don't forget the dress uniforms, they also are very skirt-like. In all likelihood it became fashionable for men to wear skirting tops some time before 2365. So popular that they became available as the "skant"-style uniform and eventually replaced the dress uniform.
The style's popularity waned and the jacket style rose as seen at the end TNG and the ...
The Franz Joseph uniform patterns from the Star Trek Technical Manual show that matching panties are a part of the normal female duty uniform. full tech manual
EDIT Some of the confusion about the fabrication method (from the comments below) is addressed here
Uhura's uniform and panties:
These uniforms were in use until the mid 2270's when the ...
In the pilot original series episodes (The Cage and The Man Trap), the science and security personnel wore blue; once the series went into production, engineering and security wore (as you said) red. (Enterprise also used this division colour scheme, but in a subtle line on the otherwise uniform uniforms).
In The Motion Picture, the security guards wore ...
Pockets and Pouches
There's no evidence that Starfleet uniforms in TNG have pockets. Generally speaking, handheld equipment for away missions (phasers, tricorders) was stowed in fitted pouches. The below screenshot (from the episode "Timescape", shows them from a bunch of angles:
It's a little hard to see (likely by design), but the pouches and uniform ...
The prison guard in Star Trek III:
Also, from TOS, The Cage we see a hat, though do not see anyone wearing it:
Finally (and possibly slightly less canonically), these must be Starfleet issue since the whole bridge crew has the same hat:
(This is a joke. This is a well-known photoshopped image. Discussion of it is on this question.)
Images courtesy ...
Picard was wearing "Casual Duty Attire"
By 2368, Starfleet had a uniform variant available to commanding officers in service aboard starships, consisting of a red command division suede jacket featuring banded cuffs and either leather or fabric black shoulders. Less formal than the standard duty uniform, this "captain's jacket" was a more casual dress ...
TL;DR: Uniform color in Star Trek indicates a person's primary role, but not necessarily their entire skillset. Many officers can fill alternate roles, despite what color they're wearing.
The uniforms typically seen in Star Trek: Voyager are the Standard Duty Uniform style that was introduced in 2369 - nearly two years before Voyager found itself stranded ...
James Tiberius Kirk has had a wide variety of uniforms over the years.
Classic Uniform Kirk as seen in many TOS episodes:
Green Tunic Kirk as seen in "Trouble With Tribbles"
Dress Uniform Kirk from "Journey to Babel"
Turtle-neck Kirk from "Where No Man Has Gone Before" (h/t to O.R. Mapper for reminding me)
Evil Kirk worn in "Mirror, Mirror"
According to the Memory Alpha article on Starfleet Uniform Code, it is explained that there seem to be some inconsistencies regarding Bajoran earings. Although, as the OP points out, Worf was able to wear his baldric, this may have been because it denotes the symbol of a Klingon's house, according to this article, which is highly significant to a Klingon. ...
You've mistaken "Operations" (those responsible for the conduct and good running of a ship or station) with "Strategic Operations" (those responsible for managing the disposition of fleets, ships and defence stuff).
Although the names are superficially similar, one is a Command role and the other is not.
The original screenplay from "Way of the Warrior" ...
It seems it may have depended upon the type of environment where personnel were primarily stationed.
throughout the run of DS9, ship-based Starfleet personnel continue to wear the old TNG uniform design. The new design may therefore have been intended as an alternate 'class' of uniform for the different environment of the space station, ...
Do headbands count as hats?
Nog was seen wearing a Ferengi-style headband (earband?) in several episodes. Not sure exactly which episodes these screenshots are from, though.
As a cadet:
As an ensign:
From Memory Alpha, here's a photo of a flag officer from the same era in the Kelvin timeline as in your original question wearing a cap in a different style:...
One character who sports multiple colours is Spock. He usually appears in science blue:
But here he is with command gold in the first episode of TOS, "Where No Man Has Gone Before":
Out-of-universe, they hadn't settled on his uniform yet. But in-universe, it's completely compatible with the fact that he is both First Officer and Chief Science ...
He was not a junior security officer before Tasha's death
The simple answer is that he was not part of the security crew before Yar died. From his career summary on the official Star Trek web site:
Starfleet Career Summary
2364 : As lieutenant j.g. in command division, assigned to U.S.S. Enterprise as relief conn and tactical officer under Capt. ...
Here is a close-up of Kirk's cluster of patches:
In the episode "Court Martial" (which is the source of the screen grab in the original question), Kirk's awards to date were listed during his trial by the ship's computer:
COMPUTER: James T. Kirk, serial number SC937-0176CEC. Service rank, Captain. Position, Starship command. Current assignment, ...
Power was a continual issue in Voyager, to the point that the crew was issued replicator rations to limit usage throughout the series.
Two examples from the page:
For example a clarinet, a complicated object to replicate, required a week's worth of rations as opposed to one ration to use the replicator one time. (VOY: "Parturition")
Tom Paris' ...
There is an out-of-universe explanation. I can't give you links on it, but during the summer between seasons 1 and 2 of Star Trek: The Next Generation, there was a convention in Richmond, VA. Now, I'm not a big convention-goer, but I have a rule: Whenever there's a Star Trek convention in my home town, I go. (So I've been to both conventions held in ...
An interesting article at TrekMovie.com describes the new uniforms in detail, and tries to shed some light on the meaning of their features based on observation of on-screen evidence.
The most important functional1 features of the Discovery uniforms are:
The blue two-piece duty uniforms are adorned with decorations of different colors2, that ...
The actual color schemes:
Command (Operations): light Green - but it looks gold on screen due to lighting.
Engineering/Administration: red - but it looked pink on screen
Command (Operations): Green - but it looks gold on screen
Engineering/Support Services: Red - different fabric used, so it looks ...
Because she hadn't obtained permission yet.
At the end of that very same episode, she asks Picard for permission to continue wearing her earring, and he grants it.
Since Worf is already wearing his baldric when we first see him, it's reasonable to assume that he has already asked for and been given an exemption that allows him to wear it.
Allowance for informal, comfortable clothing while on duty.
I believe answer has to do with formality versus informality and comfort.
As far as four seasons into DS9, we see the TNG-style duty uniforms still being used. They appear in the episode "Homefront". When Sisko returns to Earth to meet Admiral Leyton, Admiral Leyton wears the TNG-style admiral's ...
I don't think there is an in universe explanation. The Memory Alpha article on ranks agrees that there is no rank between Commander and Captain.
However, if Riker's uniform had 3.5 pips, that would complicate the plot of Cause and Effect
As Lighthart points out in the comments, we can assume that Star Fleet insignia is descended from that of the old ...
Lt. Who What Now?
Lt. Leslie has served as *deep breath*... Duty Officer, Engineer, Helm Officer, Medical Assistant, Navigator, Science Officer and Security Officer. He appeared in 57 TOS episodes and has the remarkable ability to return from the dead several times (or Dr. McCoy was drunk when he pronounced him dead) and quickly change uniforms ...
Within Starfleet, there are a few example of uniforms with pockets:
Enterprise-era jumpsuits contained a lot of pockets, suiting the utilitarian nature of Starfleet uniforms during this era.
The Next Generation TV era: Pockets are sparse in this era, though Beverly Crusher has a lab coat with pockets.
Voyager: Belanna Torres used, for most of season 4, an ...
This was the first thing that popped into my head, and Jason Baker's otherwise comprehensive answer missed it. When crew members were going on shore leave, personal items were frequently carried in rather silly cylindrical hard-shell cases:
I don't know how practical you consider this for carrying around a con. It doesn't look very comfortable.
Since he doesn't have access to a Federation replicator in his quarters (presumably Cardassian replicators don't contain the patterns for Federation uniforms) the implication is that he didn't leave himself enough time to get to a replicator station, then get changed.
Note that in the original script, he tells Dax that his plan is to get Garak to make him ...