This subject gets flogged every year or so with no satisfactory conclusion because no definitive data exists to resolve the debate. There is one thing on which N&W modelers agree: there is currently no commercial paint that accurately matches "N&W Tuscan Red" when new at the factory. Of course, which Tuscan Red are we talking about? That on passenger cars, that on the J's and K's, that on the Red Birds, on the excursion cars, etc., etc.
Many modelers have developed mixes of commercial paints which they believe are close enough to their eyes to represent they saw or see in photos. Those mixes have appeared here and in other publications. The best that anyone has been able to say is that the mix looks close to them.
It seems that until the mid-50's, passenger cars faded to somewhat of a "tomato-soup" red.
Here's one I used once, and was reasonably happy. I use ModelFlex by Badger on plastic, as it goes on nicely and is useable without thinning in an airbrush. I am using a mixture of:
If I get it a little different next time, so what? All cars fade and oxidize, so variations make it more prototypical.
I've mixed acrylic paints before (Pactra when it was available), but the latest one I thought worked out reasonably well was a gloss black basecoat with a topcoat of Polly Scale acrylic Lehigh Valley Cornell Red. After completely dry, I use glosscote over the top to give it a glossy finish. It worked well for me and seemed to closely match other cars that I painted with a mix of paints no longer available. I've used those for NW excursion models and NS business cars. I can't vouch for color accuracy other than it seems to match other models I've seen and works for me compared to various photos I've seen.
I agree with you about the Scalecoat II Tuscan red. I think it is the best N&W color match right out of the bottle. I used it to paint my S-1 sleeper. The original Scalecoat was not bad either. I painted a Pennsy car with it, and it looked too warm and rich to be Pennsy.
"Old Red" was used until around 1958. This had a much higher red content than the present red AND included a metallic particle suspended in the paint. There were a number of manufacturesr who supplied this paint to the railroad including Dupont and Dolphin Paints of Toledo, OH. Probably the closest matches are in automotive colors: Either Sherwin Williams or Martin Seynour 1975 Chrysler Vintage Red. I believe the Martin Seynour number is: 99P-2297.
In checking with a local model painter, he indicates that he used Model Felx "Milwaukee Road red" with a small amount of silver added to simulate the metallic particles.
"New Red" tended to have a purple tint to it! When #611 was painted for excursion service, New Red was used. The only automotive numbers I have been able to come up with are Dupont numbers. Pre-1975, the number was 181-24899 , "Tuscan red". Post-1975, the number is 44106, "Dark Red Metallic Enamel". I do not know how to match this with model colors.
So if you pick a Red that is more red than PRR Tuscan you would really be OK. I like Scalecoat N&W Red or Scalecoat #14 Pennsylvania Caboose (cabin car) red. As I said once before the last time the Society went through this discussion and debate: Pick the "N&W Red" you like Be Happy!!!