Guru Kian Sakhiyan is a book written by a Bhatt Sikh named Bhai Sarup Singh Kaushish in 1790 A D. In Sakhi no. 75, Sarup Singh Kaushish mentions that Dasmesh Pita undid his turban and took out a section of his under turban(KESKI) and raised it on top of His dastaar. This was called FARLA. After this some singhs also did the same and and made blue farlas.
This sakhi tells us that first first Farla was made by Dasmesh Pita Himself and Guru Sahib used His keski to do so.
Secondly, the sakhi mentions Singhs making flue farlas, which means they were wearing BLUE KESKI.
Here is a link to the sakhi:
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www.babasurjeetsingh.com]
ਸਤਿਗੁਰਬਚਨਕਮਾਵਣੇਸਚਾਏਹੁਵੀਚਾਰੁ॥