From the mid 2260s to the early 2270s, Starfleet uniforms were drastically modified from their earlier versions. The general tailoring of the "tunic and pants" uniform design worn by Starfleet service people was maintained, but with the addition of an updated collar, new rank insignia, and revised colors for Starfleet divisions and departments.
These uniforms were made of xenylon, an algae-based fabric with a special appearance not often seen in the clothing before this period. The difference in fabric appearance was considerable, as Captain James Kirk once pointed out to an illusory inhabitant of the Melkot simulation of Tombstone, Arizona. Xenylon resulted in the crew of the USS Enterprise being turned into dehydrated porous cuboctahedron solids during an encounter with the Kelvans. In addition, they had built-in temperature controls to accommodate for cold weather environments.
Command uniforms ranged in color depending on the type of fabric used. While they were generally described as gold, some of the darker variants had a distinct greenish hue, while the dress uniforms, and a captain's alternate tunic were clearly green.
Operations uniforms were a warm shade of red in most variations.
Science and medical uniforms were a mild shade of blue.
Cadet uniforms were metallic gray, woven from a special reflective thread.
The standard duty uniform was worn with standard issue black leather boots that rose above the calf and zippered along the inside. Black pants were worn that ended in length midway down the shank of the boot and terminated in a one-inch-wide flare, similar to the style worn by sailors on old Earth. The duty uniform top was a long-sleeve, division-colored tunic, with a closing seam from the collar to the left shoulder.
Rank insignia were displayed as sleeve stripes, and an assignment patch on the left breast was worn by all personnel. While the previous uniform style had a ribbed low turtleneck collar, the replacement design had a black collar with a V-shaped bottom. Black undershirts were issued, but not strictly required for everyday wear.
A short-sleeved "smock" tunic was often worn by medical personnel. It appears to have been made from a fabric similar to that of the jumpsuits and gave the user more free use of their hands than the standard long-sleeved tunic.
Additional duty and off-duty wardrobe can be found HERE.
Female crew members could choose between a standard uniform with pants, or a one-piece uniform that continued down into a short skirt over matching high cut shorts, worn with black hose and boots. Female uniform tops also had a large open collar, the seam closed right at the assignment patch. There were two variations of the female tunic: one with a black collar and another version which had the collar colored the same as rest of the tunic.