Two tones carrying a whole image — a palette of exactly two colors in conversation; also a Prism theme wearing the technique.
Latin duo (two) joined with Greek tonos (a stretching, a tone), a hybrid pairing naming exactly two colours held in conversation.
Its Greek half did not originally mean colour at all: tonos named a stretching, the tension of a string pulled taut, only later loosened into the pitch or shade a stretch could produce. A duotone holds exactly two of those stretched tones in conversation, and no more.