The "brown shade" is a representation of the color of the stigma. When used as a dye 'it is an unstable colouring agent; the imparted vibrant orange-yellow hue quickly fades to a pale and creamy yellow. The saffron stamens, even in minute amounts, yield a luminous yellow-orange colour. Increasing the amount of saffron applied will turn the fabric's imparted colour an increasingly rich shade of red.

So it could be the unfaded, originally hue that a heavy fabric like wool would be. Lighter/thinner materials may have faded quickly or not have taken to deeper hues.

Look at it as either a heavily dyed, unfaded saffron or as an ancient weathered material. None the less, this is the color that Pearse and the few Irish Nationalists selected to represent the revival back in 1900. With there being little documented history to support the Irish Kilt, the "brown shade" seems to be consistent with those that were early in its existence.