A Translation of the Seir Mutaqharin, ca. 1789
Ghulam Hussain Khan, translated by M. Raymond, 1789 They form a particular society as well as a sect, which distinguishes itself by wearing almost always blue cloaths (v), and going armed at all times.
(v) It is true that they wear only a short blue jacket, and blue longdraws: but they use likewise the yellow and the white in their turbans, as well as the blue, although by the by the latter is the general colour.
Okay I think I found it
One of most unlikely places
If you go down on sikhiwiki page on prof puran singh, the very last link is to his translation of japji and sohila-arti
In the additional references section
Bhai daya Singh rahitnama also lists yellow as a khalsa colour.
Dark grey is wrong English translation below. Original text is 'surmaye' which means dark blue or deep blue.
Daya Singh Rahit-namaClothing should be dark grey, white, yellow, or green. Do not wear red clothing.