You can see colors in this ice thanks to special light filters called polarizers. The various colors appear for two reasons: slightly different thicknesses in the ice, and tiny stresses that develop within the ice as the crystals form and collide.
Chilled by coolant from below, the glass is so cold that when you squirt water on it, the water freezes into crystals.
On some lucky occasions, you may witness a sudden, abrupt freezing of all the water on the slab. Ice crystals needs a surface or seed crystal on which to form. If none are present, the water becomes supercooled—cooled to well below the freezing point—until a seed crystal forms and ice suddenly spreads.