Damn, former user's sources totally contradict mine. I've now got no idea which is correct. The only other reference I have, which only muddies the waters more, is Richard Simkin's Uniforms of the British Army by W.Y.Carman. In the text there is no reference to the tartan and the Simkin print of the 91st shows a private and officer 1864, an officer and bugler 1874 and private and officer 1879 all wearing the same tartan trews, which appears to be either a government tartan with added red stripe or a campell of cawdor without the white stripe.
The only query I have with former user's sources are both refer to the 91st wearing Campbell of cawdor tartan after 1886, but the 91st had ceased to exist in 1881 , when under the reforms it had become the 1st Battalion of the Argyll and Sutherland Highlanders.