colour indicator

Also contains definitions of: one-colour indicator, two-colour indicator
https://doi.org/10.1351/goldbook.C01180
An indicator which is classified as one- or two-colour, depending on whether it is colourless on one side of the transition interval or possesses a different colour on each side of this range.
Source:
Orange Book, 2nd ed., p. 48 (https://media.iupac.org/publications/analytical_compendium/)