We saw the uniforms go mostly black in DS9, so I think it is meant to show how the Federation isn’t totally recovered from the Dominion war…I wish they had pushed than angle harder re: the struggle to help the Romulans, the Synths, etc. At least by Lower Decks they’re back to the more colorful uniforms!
The Cast Of ‘Star Trek: The Next Generation’ To Receive Special Lifetime Achievement Saturn Award ( trekmovie.com )