deciduous subshrub of southeastern Asia having pinnate leaves and clusters of red or purple flowers; a source of indigo dye
"The ancient garden featured an indigo, a deciduous shrub from Southeast Asia known for its pinnate leaves and vibrant purple flower clusters used to produce natural dye."
A purplish-blue colour
"The deep indigo sky signaled that night was finally falling."
having a deep purplish-blue colour
"The indigo dress looked striking against the bright yellow flowers."
In plain English: Indigo describes something that is very dark blue, almost black.
"The deep indigo sky turned completely dark as night fell."
The word comes from the Latin indicum, meaning "Indian dye," which was borrowed from Ancient Greek. It entered English in the 16th century via Spanish and Portuguese, all tracing back to the Greek term for India.