Get an ice cube, and pour a mound of salt on top of it. Pour a couple of drops of regular red food coloring on top of the salt. Wait for a minute or so. Look at the mound of salt now... it shimmers with a green color!
This phenomenon is called iridescence (ir-uh-DESS-uhnce). The molecules in the food dye that give it its red color form a thin film on the surface of the salt crystals. Further, the sodium in those crystals alters the molecules in such a way that light reflecting off the film undergoes "interference". Instead of seeing just red, the interference produces the shimmering appearance of green. Oddly, the effect is not as noticeable if you simply pour the food coloring onto a mound of salt without the ice cube.