Indigo is a colour named after the purple (blue) dye derived from the plant Indigofera tinctoria and related species.
Indigofera tinctoria bears the common name True indigo.
The colour electric indigo is used to symbolically represent the sixth chakra (called Ajna), which is said to include the third eye. This chakra is believed to be related to intuition and gnosis (spiritual knowledge).
"The quality of being an individual; individuality" related to possessing "An individual characteristic; a quirk."
Individualism is thus also associated with artistic and bohemian interests and lifestyles where there is a tendency towards self creation and experimentation as opposed to tradition or popular mass opinions and behaviours as so also with humanist philosophical positions and ethics!