Those of us who grew up in the seventies, boys and girls alike, lived in brown, orange and varieties of sludge. Seriously, if you look at DH and my childhood photos, we're in identical colour schemes half the time.
OP, I think you're projecting and assuming all over the place. You don't know the family, yet you somehow know what the daughter thinks, how she behaves at other people's houses, that she apparently compares herself to her 'girly' best friend (and I have news for you, what you wear doesn't make you any more or less female
) - and you make a very odd remark about how the 'beautifully dressed' mother is 'obsessed about avoiding anything revealing' - what on earth do you mean? Are you talking about swimsuits, or are you - surely not, given that the child is six -- equating 'girliness' with wearing 'revealing' clothes? 
In other news, I was at an after-school soft play birthday party for reception children yesterday evening. Out of the little girls I noticed, who were sitting closest to my end of the party table, one was wearing shorts dungarees with stripy tights, one was wearing jeans and a stripy teeshirt with an owl on it, one was wearing the local football club strip, one was still in her school uniform, and one was wearing some kind of princess fancy dress with fairy wings. And I have been to about a million whole-class parties all this year, and haven't noticed a pethora of little girls wearing elaborate party dresses. Isn't that a bit passé?