As for D&RGW, it was called Russian Iron or Russian Black.  But like you pointed out it was in actuallity a "green" color.  So, it was eventually referred to as Russian Green.

The original boilers were not painted, it was the process of producing the steel that created the greenish color.  Russian Iron is that process.  So in the right light it either looked green or black.

Later engines were painted to help match them to the older locomotives that had the Russian Iron Boilers. 

Only 4 of the K-27s had green boilers for a brief moment.  All 10 K-28s,  489 of the K-36s and 497 of the K-37s.  Most of the green boilers were gone by the transition to the "FLYING RIO GRANDE"

As for match, it depends on what you want.  And most records show that it varied from model to model and even which shop applied the paint in regards to Rio Grande.  Alamosa Shops even had an internal letter chastising them about their painting techniques. 

Brunswick Green is good.  Pennsylvania Green is good as well.  A shiny engine will be quite bright, but a hard working mudhen is going to have an extremely sooty boiler and will not show any green no matter what.

Josh 

Thanks for the info, Josh.  I'll try to get a small bottle of each color to test it.  I have tons of stuff that needs paint so it won't go to waste.

Peter