Han purple

Han purple and Han blue (also called Chinese purple and Chinese blue) are synthetic barium copper silicate pigments developed in China and used in ancient and imperial China from the Western Zhou period (1045–771 BC) until the end of the Han dynasty (circa 220 AD).

Azurite was the only natural blue pigment used in early China. Early China seems not to have used a natural purple pigment and was the first to develop a synthetic one.

Han blue in its pure form is, as the name suggests, blue.

Han purple in its pure form is actually a dark blue, that is close to electric indigo. It is a purple in the way the term is used in colloquial English, i.e., it is a color between red and blue. It is not, however, a purple in the way the term is used in color theory, i.e. a nonspectral color between red and violet on the 'line of purples' on the CIE chromaticity diagram. Perhaps the most accurate designation for the color would be to call it 'Han indigo', although it could also be regarded as a bright shade of ultramarine (classifying ultramarine as a color and not a pigment).

The purple color seen in samples of Han purple is created by the presence of red copper (I) oxide (Cu₂O) which is formed when Han purple decomposes (the red and blue making purple). The decomposition of Han purple to form copper (I) oxide is

Above 1050 °C, the CuO copper (II) oxide breaks down to copper (I) oxide:

...
Wikipedia