What would cause the albumen of a hard boiled egg to turn purple?
Everything you need to know: Google for "Purple Deviled eggs".  

But if you didn't pickle it, then beware.  


  Pink or iridescent egg white (albumen) indicates spoilage due to Pseudomonas bacteria. Some of these microorganisms—which produce a greenish, fluorescent, water-soluble pigment—are harmful to humans.
  Source: United States Department of Agriculture