Color the water chestnuts: Drain and dice the water chestnuts into small even pieces. Place in a bowl and add red food coloring and toss to coat. Add more until the pieces are evenly red.
Coat in tapioca starch: Add tapioca starch to the bowl and toss until every piece is generously and evenly coated in a white layer of starch and there are no wet spots left.
Boil: Bring a pot of water to a boil. Add the coated water chestnut pieces and boil until the tapioca coating turns translucent and the rubies float to the top, about 2-3 minutes. Remove with a slotted spoon and immediately transfer to a bowl of cold ice water to stop the cooking and keep them from sticking together. Drain and set aside.