indigo

This term describes a specific spectral bridge between blue and violet, often associated with the deep, saturated hue of the night sky or dark denim. It carries a sense of richness and intensity, sitting in a narrow band of the visible light spectrum.

As a noun referring to the plant or the dye, it is typically treated as an uncountable mass noun. When referring to the color as a general concept, it remains uncountable, though it can function as a countable noun when referring to specific shades or variations of the color.