Burnt ochre slip paint. It was used 'in living memory' in the North Pennines on drystone wall. Just the north facing walls and the body of the walls, not the caps or throughs. There's a good example beside the road at Halton-Lea-Gate, leaving the village, toward Alston, from Carlisle. And there's one or more railway bridges - just the north sides though. Why not? Must have looked great. My late friend remembered painting one and showed it me. 'Oral tradition'.