The Avenarius ( no "o") company was german but they had a plant in Milwaukee. Best suggestion is a sort of chestnut colour when new. Tamiya Red Brown XF64 is, I think, intended for WW2 panzers but looks about right.
Since the product was more creosote than paint, it weathered differently to your common barn red or BCR. Depending on what was underneath and how thickly it was applied and in how many coats it might tend to grey out a little, but with areas varying to an oily blackish appearance. (Look at an old creosoted fence and imagine it starting off brown rather than black)
From: STMFC@... on behalf of Mike Turner yardcoolie1968@... [STMFC]
Sent: Saturday, 13 August 2016 3:08 a.m.
Subject: [STMFC] paint color
What paint color would approximate wood painted with dried Avenarious Carbolieum? Various texts describe it as red brown or nut brown. These descriptions do not specify newly cured or aged color. TIA.