Gwen will decide on Katy's hair until she is about 10. Then Katy will decide on Katy's hair. Trust me. You're probably right, though kids have a tendancy to follow every move Gwen makes, so Katy will probably want to dress and look like mommy.
(And because Gwen does not allow me to post photos of her....if you take Helen Hunt early in her career, but with reddish-brown hair, that's Gwen)
Katy so far seems like a 50-50 blend of Gwen and myself.