A very dull rainbow
Since the Legion is composed not just of representatives from Earth, but from the entire Universe, you do rather get the feeling that the 30th century is a bit lacking in cultural diversity.
I realise that due to the prejudice of the time these comics were first published there are some parts of "the land of the free" where people would refuse to sell a comic that had a black face on it, but they could have thrown in a few pastel shades that the bigots wouldn't be offended by to at least suggest that the universe of the future wasn't almost entirely populated by white people. Of the sixteen legionnaires (guess which one is not featured on the big board here) and five subs who are supposed to represent maybe twenty different worlds, there are only two who are not white, and only one who has any physically different characteristics. And he's the one who can change himself to fit in whenever he wants.
And admittedly it's hard to tell when it's purely skin colour and you're reading a black and white reprint, but going by what I've so far read of this Showcase collection it appears that all women of the future are white Caucasians.