Sikhs adopted blue coloured dress towards the end of 17th century in the memory and honour of a group of Pashtuns

Portrait of Phula Singh Akali, 1850 (c).

A group of blue-clad Pashtuns left a lasting impression on Guru Gobind Singh (1666-1708). According to Siyar-ul-Mutakherin, once when Guru Gobind Singh (the founder of Khalsa) found it difficult to cross the territory of Sirhind, he hired Afghan (Pashtun) mountaineers in the vicinity of Sirhind to escort him to his family and promised gold for doing so. Those Afghan mountaineers engaged him to let grow his whiskers and beard to certain length, and they dressed him in blue in the style of the Afghans of that region. They escorted him with perfect safety and honour. Whenever any one asked who he was, they would answer that he was a Pirzada of theirs. It left a great impression on Guru Gobind Singh and he retained the Afghan garb in memory of that event, and he even made it henceforth the distinctive garb of his followers. He also directed his followers to grow their hair and beard. [See "Seir ul-Mutaqherin" (completed in 1780), English translation, vol-1, p-91].

The Rohillas (Indo-Afghans) usually wore blue-coloured dress in India. The author of "The people of India" (1868-1875) writes: "The "Rohilla" costume in the Deccan is a blue cotton shirt, white or blue drawers, and white, red, or blue turban, with a blue quilted jacket for cold weather."

Rohilla men, c1850. British Libray

This tradition is understandably not reported by Sikh writers of Runjeet Singh times because the Khalsa in 18th and early 19th century regarded Pashtun Muslims as their enemies and would have this tradition best forgotten. 

Banda Singh Bairagi (an anti-Muslim fanatic) attempted to suppress the blue-coloured dress [1], may be because he was irked by the fact that it was adopted in memory and honour of Muslim Pashtuns. Banda Singh had suffered defeats at the hands an Orakzai Pashtun [2] as well as an Indo-Afghan of Kasur.

A Sikh couple, 1805 (c). Source

A Bangash Pashtun in blue dress, c.1827-1843. By Imam Bakhsh Lahori, Illustrations des Mémoires du général Claude-Auguste Court, Lahore. Source

"Ruhela Pathan Seized by Jat" By Sanee, Company School, c1870-80. Source