On Architecture

2. They make a fine purple colour by treating bilberry in the same way and mixing it with milk. Those who cannot use malachite green on account of its dearness, dye blue with the plant called dyer's weed, and thus obtain a most vivid green. This is called dyer's malachite green. Again, for want of indigo, they dye Selinusian or anularian chalk with woad, which the Greeks call i)sa/tis, and make an imitation of indigo.