What is "transition" glass?

When glass manufacturers change their tanks from one color to another, the bottles that are produced between the “true” colors are called transition glass. The quality of the glass stays the same.
  

Transition glass, when available, can be a money-saving way to get into a basic bottle.  It is always sold on a first-come-first-served basis and the color is never guaranteed (as this is the very nature of transition – it varies).