Neutral red

Neutral Red (or toluylene red, Basic Red 5, or C.I. 50040) is an eurhodin dye used for staining in histology. It stains cell nuclei red. It is used as a general stain in histology, as a counterstain in combination with other dyes, and for many staining methods. Together with Janus Green B it is used to stain embryonal tissues and supravital staining of blood. Can be used for staining Golgi apparatus in cells and Nissl granules in neurons.

Neutral red can be used as a vital stain, to stain living cells. It is used to stain cell cultures for plaque titration of viruses.

Neutral Red is added to some growth media for bacteria and cell cultures. It usually comes as a chloride salt.

Neutral Red acts as a pH indicator, changing from red to yellow between the pH 6.8-8.0.

Practical Joke Uses
It's claimed that Neutral Red turns human urine red. The Wiretap Practical Joke Archive details doctoring red wine with Neutral Red to convince the subject they are urinating blood. Neutral red's tendency to turn from red to yellow between the pH 6.8-8.0 throws doubt on the validity of this claim.

Similar claims about Methylene Blue exist