"I'm not racist when I say Character X was white in the comics and should stay white, I just want them to stay faithful to the source material."
There's usually more to this argument, generally centered on the failure of the latest Fantastic Four movie (which is unambiguously blamed on casting Michael B. Jordan as the Human Torch and not on any of the other changes they made from the original comic) and on how Marvel has been so successful because they stayed faithful to their source material.
So, let's just take care of this one now, shall we? Bucky Barnes is not an orphan and "camp mascot" who meets Steve Rogers for the first time after he becomes Captain America. Bucky also did not get strapped to a missile that was launched over the Atlantic and exploded, plunging him and Cap into the icy waters. The Red Skull is not a masked Nazi handpicked by Adolf Hitler as his personal attendant. HYDRA is not created by Baron Wolfgang von Strucker, who is not a World War II era Nazi and does not wield the Satan Claw as his signature weapon. Arnim Zola is not a bio-android wearing a TV screen on his chest that shows his original face (although that could still happen, Russo Brothers!) The Falcon is not a street-smart hustler whose memories were altered by the Cosmic Cube (which is not called the Cosmic Cube and does not have the same properties or origins as the Cosmic Cube). He does not have a trained falcon. Whiplash is not seeking revenge on Iron Man for the destruction of his home village at the hands of a man wearing stolen Stark technology. Volstagg is not a comedic figure whose cowardice and incapacity in a fight is played up whenever the character appears. The Destroyer is not animated by the consciousness of a living person. The Chitauri are not shapeshifters and have not been infiltrating human civilization since before World War II. AIM is not an offshoot of HYDRA headed by MODOK. Iron Patrior is not Norman Osborn and is not a super-villain seeking to masquerade as a patriotic hero. Ultron was not created by Hank Pym. Hank Pym was never Giant Man, Goliath, or Yellowjacket. The Purple Man is not actually purple. The Abomination was not a KGB spy who bombarded himself with radiation in an attempt to become a second Hulk. Sam Sterns is not a janitor from Boise. Drax is not an undead revenant created by the forces of the cosmos to murder Thanos. Nebula is not a cosmic con artist pretending to be Thanos' grand-daughter to make use of his reputation. Ronan is not an officially sanctioned law officer of the Kree government. Korath is not a blue Kree scientist who gave himself superpowers. Rocket is not a jet-booted swashbuckler created as a therapy animal for a planet-sized insane asylum. Yondu is not from a thousand years in the future. Yondu is not a wise and noble mystic from Alpha Centauri. Yondu does not use a bow with his arrow. Yondu is not disinterested in consuming human flesh. Star-Lord is not...oh, let's just say that Star-Lord is not anything Star-Lord ever was in the comics and leave it at that, okay?
In short, Marvel has taken vast liberties with every single aspect of their source material, and 99% of them have been greatly adored. It is significant that the only ones some people even notice, let alone take issue with, are the ones that change the skin color of their favorite characters. Let's not pretend otherwise, okay?
(Feel free to link to this post any time someone is making this argument online and you don't want to just retype it all, by the way.)