Indigo 

Indigo - a natural dye of the plant origin, produced during the fermentation process of the plant, called in Japanese ai (Japanese indigo Persicaria Tinctoria). It allows obtaining a broad and varied range of colors, from off-white, through all shades of blue, to the deep navy-blue, which sometimes may seem black. It is particularly popular as a fabric used in the summer kimono - yukata, because it resembles the attributes of a summer sky and the sea. 

Once popular as the dye for children and samurai clothes due to its insect repelling and healing properties (soothes skin injuries and sores).