If i have dream hall in the battlefield, can I cast an eldrazy by just discarting an artifact?
{dream hall}
When casting an eldrazi with a color, {broodwarden} for example, you may discard acard of the appropriate color (green in this case) instead of paying the casting cost. When casting a colorless eldrazi ({emrakul the aeons torn}), you cannot use this effect because colorless isn't a color, and therefor can't share a color with another card.