Skeeter will hate this entire post.

UNIFORM CODE
Wherein I will discuss the uniform traditions of Star Trek, especially with an eye on how the various traditions were combined in the prequel series Star Trek: Enterprise. This discussion will not include the original series movies, as both styles of uniforms represented therein followed entirely different color and rank insignia schemes.

{23rd Century - Star Trek (the original series)}
Colors
Gold really more of a greenish yellow - Command, Navigation

Red - Engineering, Security, Communications, JAG

Blue - Medical, Sciences

Ranks
For officers, ranks were determined by shiny gold stripes on the lower sleeves. Ensigns and crewman both had blank sleeves.
Commodore - two solid stripes with a gold field between

Fleet Captain mentioned, but no uniform was ever seen

Captain - solid stripe, dashed stripe, solid stripe

Commander - two solid stripes

Lieutenant Commander - solid stripe, dashed stripe

Lieutenant - one solid stripe

Lieutenant (junior grade) - one dashed stripe this is a conjectural guess

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{24th Century - Star Trek: The Next Generation, Star Trek: Deep Space Nine, Star Trek: Voyager}
Colors
Red - Command, Navigation, JAG

Gold - Engineering, Operations, Security
Essentially, the roles of gold and red were reversed, though JAG (Judge Advocate General, legal services) stayed red, thus becoming more of a command function, which seems logical.

Blue - Medical Sciences, Communication

Ranks
Officers ranks were determined by round pips displayed on the collar. Enlisted crewmen for the most part had no rank insignia. The system of gold and black pips is identical to the system of full- and half-stripes emplyed by the United States Navy in the real world, with the exception of the admirals.
Fleet Admiral - four gold pips on a black field, trimmed with a gold border

Admiral - three gold pips on a black field, trimmed with a gold border

Vice Admiral - two gold pips on a black field, trimmed with a gold border

By the 24th century, the title of Commodore had lamentably disappeared from Starfleet.

Captain - four gold pips

Commander - three gold pips

Lieutenant Commander - two gold pips, one black pip

Lieutenant - two gold pips

Lieutenant (junior grade) - one gold pip, one black pip

Ensign - one gold pip

Chief Petty Officer no positive identification
For the last four seasons of DS9 Chief O'Brien's collar featured a gray patch at which we never got a terribly good look.

Senior Chief Specialist - one black pip
This arrangement was seen on various transporter operators referred to as "chief" and also Chief O'Brien at the time he was identified as a Senior Chief Specialist.

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{22nd Century - Star Trek: Enterprise}
At the time of the adventures of Captain Archer and his crew, the Federation - which Captains Kirk, Picard, Sisko, and Janeway faithfully served - does not yet exist. Starfleet is a service of the United Earth government and employs a mix of 23rd and 24th century traditions.
Colors
Gold - Command, Navigation

Red - Engineering, Security
The roles of red and gold were returned to their original series configuration, which makes sense.

Blue - Medical, Sciences, Communication
Curiously, blue is used for communication, as in the 24th century, even though it was red in the 23rd century. I personally believe this was done because neither Sub-Commander T-Pol - the Science Officer - nor Dr. Phlox - the Medical Officer - is a member of Starfleet. If Ensign Sato - the Communications Officer - did not wear blue, non one would.

Ranks
As with communications, here the 22nd century bears more resemblence to the 24th than the 23rd century, though it is not identical to either. Consistent with both other centuries, enlisted personnel have no visible insignia.
Admiral - one silver stripe, two silver stripes with a silver field between, one silver stripe

Commodore - two silver stripes

Captain - four gold pips

Commander - three silver pips

Lieutenant Commander - two silver pips, one black pip this is entirely conjectural, as we haven't seen one

Lieutenant - two silver pips

Lieutenant (junior grade) - one silver pip, one black pip again, entirely conjectural

Ensign - one silver pip