Purple Colours. Painters in oil and water colours produce various shades of purple by mixing certain red and blue pigments. For work in oil French ultramarine, often called French blue, is mixed with vermilion or some madder red (madder carmine is best), or one of these reds with cobalt blue if a pale purple is wanted. For permanent purples in water-colours the same blues are used; but one of the madder reds, not vermilion, should be mixed with them. A much richer purple than any of the above mixtures will give is produced by Prussian blue and one of the lakes from cochineal—viz. carmine or crimson lake—but it is not permanent. This purple, as well as that obtained by mixing Indian red with indigo, also fugitive, was much used by water-colour painters in past years. Purple madder is the only simple purple pigment available for the artist which is durable, and it is unfortunately costly. All purples are changed to neutral and gray tints by the addition of any yellow pigment. For house-painting moroan lake with a little French blue gives a useful purple; but some of the above mixtures also are occasionally used.
There are several ways of dyeing textile fabrics of a purple colour. The most famous of all ancient dyes was the Tyrian purple, which is said to have been discovered at Tyre many centuries before the Christian era. Among the Romans this colour was exclusively employed for dyeing the imperial robe. It was obtained from shellfish belonging to the genera Murex, Purpura, and Buccinum; at least it has been supposed that it was prepared from one or more species of each of these. The colour was so costly that in the time of Augustus one pound of it sold for what would amount to £36 sterling. About the year 1851 what is believed to be the same or a closely similar purple was obtained from uric acid by a peculiar treatment (see MUREX; DYEING, Vol. IV. p. 139; and PHŒNICA). Archil (q.v.) seems to have been the only simple purple dye known in the middle ages. Purple of Cassius is a compound of gold and tin used in colouring Glass (q.v.), and in porcelain and enamel painting. It was discovered at Leyden by Andrew Cassius about 1683. A preparation of this colour was formerly used for painting miniatures in water-colour; but for this purpose purple madder, being cheaper, has taken its place.