Welcome aBoard, Pam! Hmm--our previous discussion didn't get as far as I'd thought it had:
http://wordsmith.org/board/showflat.pl?Cat=&Board=words&Number=27664; nor did the link included in it.
I've always thought it had to do with the obviousness of having a different parentage. There is a song which I'm not remembering much of right now including the title, but I believe it was about a red-headed child, and Papa had been absent for a while; I do remember one line clearly: "...And the milkman had red hair".

Why don't you try going to Information and Announcements and looking in Max's Useful Language Links? I'm sure there are some word and phrase origin sites listed there.