Right now there are two prevailing theories about why Carol Danvers’ suit is green in these Captain Marvel set photos.
The first is that because green films better, there will be some tinkering in post-production to alter the suit’s appearance.
Captain Marvel is going to feature several Kree characters, so it’s possible that after obtaining her special abilities, Carol will initially put on a standard Kree uniform and modify it to resemble what she wears in the comics either towards the end of Captain Marvel or in Avengers 4.
The second is that Carol is wearing a prototype version of her costume, and that later on she’ll rock the red, blue and yellow coloring, which we’ve already seen in concept art from last year’s San Diego Comic-Con.