Storminess at sea, as in the sky, can be purple-inked.

‘Beyond the ragged horizon of the purple-grey sea…’

Source: Henry Williamson, Tarka the Otter: His joyful water-life and death in the two rivers, illus. C.F. Tunnicliffe (Harmondsworth: Puffin Books, 1976 (1927)), p. 105