Neutral Red, a nitrogenous pH-indicator with a pKi of 6.8, is an indicator for the internal acidification of thylakoids. Neutral Red stains lysosomes red[1].
As cells begin to die, their ability to incorporate neutral red diminishes. The neutral red is also used to stain cell cultures for plate titration of viruses[2].
References:
[1]. Winfried AUSLANDER, et al. NEUTRAL RED, A RAPID INDICATOR FOR pH-CHANGES IN THE INNER PHASE OF THYLAKOIDS. FEBS LETTERS. 1975 Nov.
[2]. Ellen Borenfreund, et al. A simple quantitative procedure using monolayer cultures for cytotoxicity assays (HTD/NR-90). Journal of tissue culture methods volume 9, pages7-9 (1985).