I use probably 10% or less ink per alcohol. It mostly turns grey. IF you go too high on the ink, it gets more black. You can always go back and add another coat of stain to darken the wood, but its almost impossible to remove stain once applied.
One thing I have noticed that some modelers do not do,
most of the wood used along the D&RGW has a red-brown tint to it unlike most of the white woods used in modeling. This applies to both structures and rolling stock. Start by staining it with a very thinned redish brown tint (your choice as anything will match at some point in history), then INDIVIDUALLY color the wood at different percentages of india ink, or use some kind of glossy black stain for structures...
If you look at Russo's picture, it looks like they stained them in oil or Creosote, but not very well. Every board has a different shade of brown, and a different consist of black. Most even have both colors that fade from one to another.
Of anything I have learned about modeling the Rio Grande (Western and Southern), anything is possible. If you see something similar that matches your model, even if not the same prototype, no one can really say it was never that way!!!
Casey