I would imagine tartan, I'll have a look at some of my books tonight. I've got a couple of books that show pipers from just before the great war, the new Osprey 'Vanished Armies' Book and the British Army List 1914 both have good colour drawings of some of the scottish regiments. The cords and cover are removable (my pipes dont have a cover on them at all at the moment so it just a plain Sheepskin bag

) So they may well have changed in the war to anything that was to hand.