Sean @ Make: Online writes:

I love it when people get really, really serious about the preparation of drinks. Beverage and spirits writer Camper English is such a person. He has done some interesting experiments with freezing large pieces of crystal-clear ice at home (he reports boiling it first does not work), and freezing bottles of liqueur inside large blocks of it. His latest brainchild is this ice cube with a pocket of food color frozen inside, which is slowly released as the ice melts. The potential permutations on this trick are numerous: different colors in different cubes, different colors in the same cube, colors that react with citric acid in the drink and fade awhile after being released, etc., etc.