I'm actually fairly certain Ragnar even referred to the Azadi markings as tattoos at some point, so I don't know if that's a deal breaker, as Flo said above.
As far as a green mom and a white dad having a darker skinned baby... A.) It could be that just one of them is his parent. B.) She has green hair and he has dark hair, and yet they have a blonde daughter. C.) Etta clearly isn't human, so...we can really only hazard guesses as to how her genetics work.
And also magic.
Still not sure I believe Kian is related to Etta and/or Magnus, but I love playing devil's advocate.