Charles,
  There is one "this is how it worked" relative to paint ... before ... color film.  And that is that
many (dare I say "most"?) RRs mixed their own "box car red", a batch at a time, in the paint
shop ... by mixing linseed oil with pigment (usually some form/from some source).  And, of
course, the local shop didn't always do things the same way every time or between different
shops.
  Yes, a lot of paint was purchased pre-mixed - but even there the colors varied in those
early years (i.e. before color film - and even well beyond the arrival of color film)..  
  Soooo - was the freight car reddish brown "pretty much the same for the same RR".
Yes, it was.  Because the RRs tended to buy from their sources (mixed or not) and
those sources tended to provide consistent product.  And yes, the Pennsy color is
different ("more red") than the color for the SP ("more brown" - expecially when 
compared to Pennsy freight car reddish brown).  Ad infinitum.

  I'm lucky - I weather everything.  And I don't use the exact same process for
weathering on every car (not even the same mixes of weathering colors).  My
goal is simple - if you look at a yard full of cars they will look "all the same" -
but when you look at individual cars (and especially when you compare two
cars side-by-side) you will see differences.  Some cars get little more than a
light dusting coat - some are heavily weathered ... and everything in between.

                                                     - Jim in the PNW