George Field, Chromatography; or, a Treatise on Colours and Pigments, and of Their Powers in Painting, &c. (Field, 1835, p.132)

Talking about the use of purple to harmonize ‘the broad shadows of a bright sunshine ere the light declines into deep orange or red’, George Field (1777–1854) claims that ‘Girtin, who saw Nature as she is, and painted what he saw, delighted in this effect of sun-light and shadow’. In an earlier section of the book Field looked at the use by artists of indigo for blues and its deletirious effect on watercolours as is so often seen in the case of Girtin's work. 

INDIGO, or Indian Blue, is a pigment manufactured in the East and West Indies from several plants, but principally from the anil or indigo-fera. It is of various qualities, and has been long known, and of great use in dyeing. In painting it is not so bright as Prussian blue, but is extremely powerful and transparent; hence it may be substituted for some of the uses of Prussian blue. It is of great body, and glazes and works well both in water and oil. Its relative permanence as a dye has obtained it a false character of extreme durability in painting, a quality in which it is nevertheless very inferior even to Prussian blue.