Meaning of Congo red

Con'go red'

Pronunciation: [key]
— Chem. Chem.
  1. a water-soluble powder, CHONSNa, one of the Congo colors that is not acid-fast or lightfast: used chiefly as a dye, biological stain, and chemical indicator.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.