Greg responds...

Pretty much. I don't want to say all Martians look alike, but frankly to a human they probably would. Even distinguishing males from females would be tough to the untrained human eye. (Then again to the untrained Martian eye, I'm sure all humans tend to look alike too.) If you saw J'onn's true form, the obvious main differences between him and M'gann are that J'onn's green and bigger.

The only real difference between Green, White and Red Martians is skin color - which is a pretty superficial difference, of course, genetically, but a significant difference historically and culturally on Mars.

Of course, on Mars-16, the White Martians aren't a separate species - and they're only a different "race" if you are using the word "race" the way we Earthlings do to distinguish Caucasians from Africans from Asians. And certainly, the White Martians are not monolithically evil.

Oh, and for the record: M'gann IS J'onn's niece. She didn't lie about that. M'gann's mom (i.e. J'onn's sister) is green. Her dad is white. Most of M'gann's siblings are also green, though she has one white brother.

Wow, why did I suddenly give all that away?