It's an origin story. Looks like they are taking a page directly out of the comics. Carol Danvers is an air force pilot in the 90s, meets the Kree captain Mar-vell becuase (....reasons, actually I know just don't wanna spoil it), and gets recruited to the Kree fighting group (Strikeforce). That's why she's wearing a green uni at first. The movie will probably explain her Kree origins and her other human side (i.e., her past). Nick Fury will either find or recruit her to fight the Skrull invasion, get her the awesome red and blue uni, kick ass, and then go into Avengers End Game.