Corrupted Grafstone
Shadows over Innistrad — rare
Artifact
Corrupted Grafstone rulings:
2016-04-08: Colorless cards have no color, so you can’t add to your mana pool this way.
2016-04-08: Players can’t respond to mana abilities, so no player can take any action to stop you from getting the color of mana you expect to produce once you announce the ability.