"ALL glass pipes are fumed with Silver or Gold
this is how you get the color-changing appearance, however some glass artists will "fume too heavy" for the colors to change
It requires a skilled lampworker to apply to proper amount of silver or gold to make the piece "color-changing" without becoming opaque or milky, but instead producing a iridescent array of colors including blues, greens, pinkish-reds, purple, orange and more."

- from http://www.iblowglass.com/

Basically when the glass is blown, it's fumed with certain materials like Silver,Gold or Mecury. Different materials...give different colouring.