This is taken from the current publication, "The Royal Regiment of Scotland (Scots) - Dress Regulations", Page 58 of 64.

Hackles
The hackle (or “Vulture’s feather” as it was termed) was originally an aid to identification in battle. Different coloured hackles were used to identify different companies: white hackle for right of the line, green for light infantry company, red and white for companies in the centre of the line. There were many Regimental variations. The red hackle worn by 3 SCOTS originates from an action of the 42nd at Geldermalsen on 5th January 1795. Later that year, on the King’s Birthday, there was a parade at Royston, Hertfordshire, when a Red Hackle was distributed to every man on parade. But it was not until 1822 that an order from the Adjutant-General confirmed that only the 42nd would have the privilege of wearing the Red Vulture feather in their bonnets. Red Hackle Day is still celebrated by 3 SCOTS. The blue hackle worn by 4 SCOTS originates from a visit to the 1st Camerons in France in December 1939 by King George VI when he gave permission to wear a royal blue hackle in their bonnets. The white hackle worn by 2 SCOTS originates from permission granted to the Royal Scots Fusiliers for their services in the South African War of 1899 – 1902. 1 SCOTS and 5 SCOTS were granted permission to wear the black hackle and green hackle on formation of the Royal Regiment of Scotland. The hackles identify the battalion in which an officer or soldier is serving or last served.




Note the capbadge and the hackle together.

Regards

Chas