The process can repeat any number of times, as long as two cards that share a color are milled each time.
04-10-2004
wotc
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.
Random a card of Magic: The Gathering (MTG), but you can also roll the D20 dice, random a Planeswalker, have fun with the random rules and more.