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".
= (mana cost or alternative cost) + (additional costs and cost increases) - cost reductions
= discard a card that shares a color with the spell
117.6. Some mana costs contain no mana symbols. This represents an unpayable cost. [...]
117.6a [...] If an alternative cost is applied to an unpayable cost, including an effect that allows a player to cast a spell without paying its mana cost, the alternative cost may be paid.
Just in case there's any doubt, Living End is black.
204.2. An object with a color indicator is each color denoted by that color indicator.
(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.)