Color Identity

[Noblellama]

We have all accepted that {Elbrus, The Binding Blade} has a color Identity of {B} due to {Withengar Unbound}

but why, is there a rule im missing that says card boarder also dictates color identity because there is no mention of {B} mana symbols on Withengar...

I know the knew Avacyn will be {W}{R} but why?

[Kaylesh]

The double sided cards have a color marker on the left side of the card. (Just before the type text to be precise). This is what gives it color.