Here's a link to the details from the Register of Tartans:
http://www.tartanregister.gov.uk/tar...aspx?ref=10124

The Red Hackle tartan has been designed as a tribute to the men and women of The Black Watch, both past and present. Originally used to identify troops in the midst of battle, there is doubt as to exactly when the Black Watch's unique Red Hackle was adopted by the Regiment. Whether it was first used during the American War of Independence or later during the battle of Geldermeisen campaign on January 5th, 1795, what is beyond dispute is that in 1822 the Army's Adjutant General confirmed the unique right of the 42nd Regiment (Black Watch) to wear the Red Hackle in their bonnets. In memory of Geldermeisen, the 5th January subsequently became the regimental day, and is now known as Red Hackle Day. The new tartan is based on perhaps the world’s most recognised textile design, the famous Black Watch tartan, to which a series of overchecks in two shades of red has been added. It was selected from a number of designs submitted to the regimental headquarters at Balhousie Castle in Perth. A proportion of all sales of the tartan will be donated to The Black Watch Heritage Appeal.