Aren't there white martians? So WTF is this talk of whitewashing if they have numerous colors?
There were green, white, and I believe yellow martians, but for them it was more than skin deep. In the comics, different kinds of martians actually had different physiologies to some extent. The fuss about white washing is that Martian Manhunter's favored shape in maybe the comics (don't know for sure) but definitely in various cartoons and the Supergirl live action show, is of a black man.
It becomes an issue of representation. I don't know whether your average black comic fan feels like they have more representation because Martian Manhunter chooses to take the shape of a black man, though he's not human and has green skin in his natural form. But I'm not black, so I leave it to people who are to feel about it how they feel about it. *shrug*
Personally, I always liked Carl Lumbly's voice as Martian Manhunter and he is a black man. His voice for the character sounds...I dunno, vaguely African and I liked that in the character. I can't really imagine Willem Dafoe's voice when I think of a CGI or face painted MM.
Guess someone already adressed it.