Marvel Cinematic Universe Theory: Black Widow Is A Skrull
4. Her Constantly-Changing Appearance
The Skrull race's primary superpower is the ability to shape-shift, assuming the appearance of anyone they desire. Natasha herself is no stranger to changing her looks either, having sported numerous different hairstyles/hair colours since her MCU debut in Iron Man 2.
In that first appearance, her hair was long and brown, going from slightly curled at the bottom - in her 'Natalie Rushman' disguise - to fully curled and frizzy in her costumed Black Widow scenes.
Then, in The Avengers, her hair was cut short and dyed a much brighter red/ginger colour.
In Captain America: The Winter Soldier, her hair became a little lighter, much straighter, and was grown down past her shoulders, and in Avengers: Age Of Ultron, her hair returned to its Avengers 1 appearance, only a little lighter and a bit curlier towards the top.
In Civil War, her hair once again went down past her shoulders, while Avengers: Infinity War will see the character with a mop of short blonde hair, continuing her extensive list of ever-changing hairstyles.
The thing is, there doesn't seem to be an important story reason for any of these changes (besides her new Infinity War look, which is likely a result of her busting out of the Raft and going on the run, thus needing to alter her appearance).
So... what's the point of all this? One possibility is that this is all just foreshadowing - a reference to the appearance-changing ability possessed by the Skrulls. Black Widow is the only Avenger to drastically alter her hair in each subsequent movie, so this is certainly something that's worth noting.
Additionally, the character has been undercover/in disguise on numerous occasions: in Iron Man 2, she pretended to be Tony's personal assistant; in The Avengers, she pretended to be captured by Russians, and later pretended to be shocked by Loki's harsh words; and in Winter Soldier, she disguised herself as a World Security Council member by using a high-tech face-mask.
Again, these could all be nods to the shape-shifting power of the Skrulls. Granted, master of disguise and deception is a part of Black Widow's "specific skillset", but what if this part of her is a hint that there's more to the character than meets the eye?