Sentence:  It would be difficult to exaggerate the value of this addition to the artist's palette. The colour of <WSD>viridian</WSD> is a very deep bluish green of great purity and transparency. It furnishes, with aureolin on the one hand and with ultramarine on the other, an immense number of beautiful hues, adapted to represent the colours of vegetation and of water. 
Meaning of the word marked with <WSD> tags:  A bluish-green pigment made from a form of chromic oxide.