Indigo dye gets darker with each dyeing.

In Japan, there has long been a culture of calling the deep color of indigo dyed cloth "dark naby = victory color."

This color was considered auspicious for samurai during the Sengoku period, and was used on armor and flags in prayer for victory.

This spirit has been passed down to the present day and is said to be one of the origins of the "Samurai Blue" uniform worn by the Japanese national soccer team.

Japan Blue is a symbol of Japan. 

"Churasanguwa | Deep Blue" by Okinawa Blues Store. The color of victory, which is its roots, is layered on the dark naby of Ryukyu indigo.