I think the title is self-explaining :)
Living End has a mana cost of "unpayable".
Dream Halls introduces an alternative cost of "discard a card that shares a color with the spell".
Just in case there's any doubt, Living End is black.
(It had a characteristic-defining ability when it was printed, but it now has a color indicator. Characteristic-defining abilities work in all zones, so the answer would have been the same before the change.)
It says "Rather than pay the mana cost for a spell". Since it doesn't have a mana cost it has no effect on cards with no mana cost.