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 blue. 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.
I read about it somewhere in his biographies
I just remembered because I was looking for it some years ago, it's interesting because it is one of the first translations of sri japji Sahib, that too by a sikh instead of maclauffe or trumpp