Others with more knowledge will, hopefully, chime in - but the 'buff' color you describe was pretty common on wood-frame depot structures like Clear Creek, Mitchell, Pekin, and a few others. As we all know, the Monon tended to be a pretty low-buck operation for most of its independent life, and I always suspected that the most common color for structures was "whatever paint could be bought in bulk the cheapest."
Mark J
Columbia MO