I've seen the odd little boy in hand-me-down white skates at my second rink, but never at my home rink. At rink2, they also have a variety of hire skaters, including black skates, so often see lts girls in black skates (or beige or blue or white!)
It isn't an issue. I notice it in the little boys, but think it's fab that they're so keen to skate they'll wear any skates they can.
The colour skates a kid wears doesn't matter. It does not make a difference to his or her future sexuality.
BUT some small minded people will always make life difficult for people who don't conform to what they think is right. And a six year old shouldn't have to put up with it for something so easy to change wth some cheap boot covers or tape. If an adult or teenager wants to make a statement thats great, but its kind of hard to explain to a little kid.
I would warn the parents that some kids and their parents may comment on their sons skates, suggest boot covers, but emphasise that it really doesn't matter that he has white boots. I wouldn't say anything to the kid himself though.