Grindstone

Oracle Text

3, T: Target player puts the top two cards of his or her library into his or her graveyard. If both cards share a color, repeat this process.

Card Rulings

10/4/2004 If the two cards are both colorless, then the effect does not repeat. It only repeats if both cards have at least one color and at least one color is shared between them.
10/4/2004 The effect repeats as many times as it takes to meet the end condition.