I call it the Bionic Six rule:
http://en.wikipedia.org/wiki/Bionic_Six
These superheroes never wore masks and yet, the villian of the series consistently failed to recognize them in their civilian form.
I remember reading a thesis once on how people's eyes are drawn to clothing, not faces.
This would be doubly true of superheroes, who wear bright, flashy clothing.
So, when they're not wearing their super costumes, they're just ordinary everyday people.
And nobody really ever looks at the average ordinary person and says (to choose Violet as an example), hey, she's got long black hair like that Incredibles girl, are they the same person?
The color scheme also works against that, since Violet favors dark coloration and purple, if she's not wearing bright red and orange, it throws off the human visual recognition system.
Granted, as I cited with Violet and Dash, if someone who knew you very well happened to see you, they'd make the connection very fast who you were!
And I wonder, why have Violet and Dash never recieved Supernames?
In battle, their parents always call them by their birthnames.
Wouldn't that be a dead giveaway?