suō 蘇芳

A purplish-red dye senryō 染料 color name, and the plant from which it is made. The su plant sappanwood (Caesalpinia sappan) grows in areas of southern Asia, India, Thailand and Persia. It has been imported to Japan since the Nara period for use as a medicine and a dye. The red centre of the trunk is crushed, and lye added as mordant. The addition of iron changed the dye color to deep purple. 

In the Edo period suō was used to make a purple dye which was called nise-murasaki 偽紫 (false purple), in contrast to the true-purple dye *shikon 紫根, made from the *murasaki 紫 plant.