A page from the "Causes of Color" exhibit...
Why are things colored?
Why is the sky blue? Why is fire yellow? What causes the vivid colors in sapphires or emeralds? What do diamonds and rainbows have in common? Is there a connection between fall colors and flamingos?
It has been proposed that all the colors in the universe originate from a mere fifteen fundamental physical causes. These causes appear over and over, lending color to the world around us. Some common causes seem logical - for example, both light bulbs and candles are colored by incandescence - and others are surprising - did you know that the colors of peacock feathers and bubbles are both caused by interference? Very small additions of an impurity can transform transparent corundum into a gorgeous red ruby. The same mechanism makes emeralds green and produces the color in some glazes and enamels.