Quote Originally Posted by clogz
I think it is a dye, bromocresol, that turns blue when it is exhausted.
I think you are right. I remember an indicator "brom-cresol-green" which had a light greenish endpoint just before it turned (what I would call) purple - very much like indicator stop bath if reacted with excess developer. I never exhaust my stop (LOL), actually I do try to keep it fresh. After all it about the cheapest material in photography.