I think that the reason that I think of a black, charcoal grey, and a check tweed Argyll as being of different levels of formality is that I unconsciously equate them with various sorts of "Saxon" dress:

black barathea Argyll = tuxedo
charcoal grey Argyll = formal dark grey suit
check tweed Argyll = plaid tweed sportcoat with contrasting slacks

But evidently this line of thinking would never occur to a Scot, as I understand? It's good to know.