Corrupted Grafstone

Text:

Corrupted Grafstone enters the battlefield tapped.
: Choose a color of a card in your graveyard. Add one mana of that color.

Rulings:

• 4/8/2016 Colorless cards have no color, so you can’t add <img src="/Handlers/Image.ashx?size=small&amp;name=C&amp;type=symbol" alt="Colorless" align="absbottom" /> to your mana pool this way.
• 4/8/2016 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.