Earlier today, set photos for Avengers 4 arrived online that showed Brie Larson as Captain Marvel. While we've known for a while now that she'd be playing the cosmic Avenger, we'd never actually seen her in the costume.
While the first photos show a Carol Danvers who looks the same as her comic book counterpart, she also doesn't, with the most striking difference being that her costume is green, silver and grey instead of red, yellow and blue. But, despite the fan theories, this likely hasn't been done for purposes of CG. In fact, it most likely has to do with both her Kree-based origins and the possible plot of Avengers 4.
Carol didn't get the Jamie McKelvie-designed costume most fans expect to see on the big screen until her solo comic in 2012 by Kelly Sue DeConnick and Dexter Soy. In the series' inaugural issue, Carol ditched her old look for an attire that calls back to both Mar-Vell, the Kree man who wound up giving her the powers she now has, and her Air Force background.
Initially, Carol was hesitant to take on the title of Captain, and was still going by Ms. Marvel, hence the red sash around her waist. It took goading from Captain America for her to finally accept being the new Captain Marvel since Mar-Vell died saving the Kree from the power of the Phoenix Force. By embracing the title of Captain Marvel, she's also made sure to honor Mar-Vell's memory by having the same symbol on her chest that he did on his costume. Carol's militaristic look has stayed fairly consistent over the subsequent years; initially, she had a helmet that would wrap around her head at will, though that and the sash seem to have disappeared in recent years.
As for how it may tie into the plot of Avengers 4, well, since Carol's solo movie is set into the '90s, it's possible that whatever happened to her in that time has landed her in this new alternate reality that the Avengers may find themselves in. Additional set photos for the upcoming MCU film have shown that something is very messed up in the aftermath of Infinity War, meaning that the Avengers may find ourselves in an alternate timeline created by Thanos when he gets his hands on the Infinity Stones. And with Captain Marvel's solo film releasing just two months before Avengers 4, it's conceivable she'll be drawn into Thanos' orbit prior to joining Marvel's massive ensemble film.
It wouldn't be out of the question for Carol to have acquired Mar-Vell's powers in the '90s, but wind up as an agent of the Kree instead of a full time superhero. Green is the color typically associated with Kree Captains in the comics, as introduced in the very first appearance of Mar-Vell (who factors into the Larson-helmed film) in the '60s. Depending on when and how she'll arrive in the present day, she'd be affected by Thanos' reality warping in the same way that the other Avengers seem to be, and could conceivably be wearing her Kree uniform for any number of reasons.
We won't find out for certain until 2019 with Avengers 4, but one thing we do know is -- there's no way this is Captain Marvel's final look.