In the Star Trek: Deep Space Nine episode "Rapture", the outfits of the crew changed from colour to grey with a little colour underneath. Why did the outfits change?
From Memory Alpha.
This was the first episode of Deep Space Nine to feature the grey-on-black Starfleet uniforms with the division color undershirts created for Star Trek: First Contact, which would also be used for the remainder of the series. Sisko also wears a captain's vest/waistcoat, as worn by Jean-Luc Picard in that movie. The uniforms had been held back in production so that they would not be seen until an episode that aired after the official release of the movie. Noticeably, Avery Brooks' tunic does not appear to fit him correctly, making correct positioning of his combadge difficult. For the duration of this episode and part of the following one (DS9: "The Darkness and the Light") he wears the combadge on the grey portion of the tunic instead of the black; the combadge was positioned correctly in all subsequent episodes. Furthermore, Sisko does not have the red stripe around his cuffs that all the other uniforms do. Bashir's "Does my uniform look brighter?" line was a not-so-subtle way of drawing the audience's attention to the new costumes. Besides all the admirals' (and Whatley's aide's) uniform which had not been changed yet, some officers of the lower ranking staff are still wearing the previous uniforms. Unlike the DS9 crew, the crew on Star Trek: Voyager continued to use the old Starfleet uniforms, due to being stranded in the Delta Quadrant.
The Star Trek: Deep Space Nine Companion (p. 407) notes that all of Deep Space Nine's stock of the older uniforms were sent to Voyager to use.