Can I cast cost-less cards like Living End with Dream Halls?
Yes.

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".

total cost
= (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.)