Classic-Castler Jens (Schlimps on Brickshelf) presents another cool creation that uses rare-colored parts — a Nazgul, or Ringwraith, from The Lord of the Rings:
According to several members of my local LEGO club (SEALUG), rare-colored parts are likely the result of a quality control step in the manufacturing process known as a “color test.”
When The LEGO Company performs a color test, they create a limited run of a LEGO piece in a specific color to help determine what kinds of stresses the acrylonitrile butadiene styrene (ABS) can endure in that particular color. For example, a clear minifig was created some time ago, and the speculation at our recent meeting was that this better revealed the kinds of discoloration (due to friction or stress) that might occur where parts are joined together.
If you’re more familiar with the LEGO manufacturing process and I’ve gotten something wrong, feel free to correct me in a comment!