Coloring dry ice is an interesting and entertaining experiment for all ages. Although you cannot directly apply color that will absorb into or stick onto the ice, you can create an exciting billowing cloud of colored smoke by either pouring food coloring onto the cube or dropping it into a glass filled with dry ice. Dry ice does not liquify, so when liquid comes into contact with it the ice undergoes sublimation -- or turns directly into gas. The dye in the food coloring blends during this process, adding the colorful effect to the smoky gas.
Read more : http://www.ehow.com/how_7421620_color-dry-ice.html