One thing you might want to avoid saying is "Wow, you've really gone from Samantha to Serena."
The woman in question might not be totally familiar with "Bewitched." Or, if the woman is someone you do not know well, she may simply misinterpret what you are saying.
I used to think showing interest in women's hair was a winning strategy because, often, it's something they really care about themselves. But it's a minefield, my friends.