Grindstone

{1} · Artifact

{3}, {T}: Target player mills two cards. If two cards that share a color were milled this way, repeat this process.

Motor: se resuelve a mano

Rulings (2)

The process can repeat any number of times, as long as two cards that share a color are milled each time.
Wizards of the Coast · 2024-04-12
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.
Wizards of the Coast · 2004-10-04