See the illustrations here:
https://nycshs.files.wordpress.com/2014/07/pages-from-1981q2the-k-11s.pdf
Note first that in most of these shots the lagging is obviously weathered to a lighter appearance than the smokebox! i do not know if this is a film or photo-filter artifact, but it can be clearly observed in pictures from before WWI and 1952.
This goes hand-in-hand with a clear distinction between relatively glossy boiler clothing and dull smokebox finish. This to me strongly supports the 'dirty graphite' argument that has been made -- note that in the builder's photos the smokebox is bright graphite, which may indicate the method that was used at the time (and why it might weather so quickly both dull and black).
Even if you did 'color' in grays and blacks to match the tonal range of the non-color photos, you couldn't be too far away from their appearance 'in real life' -- which would have been in blacks and grays...
That is great info, thank you!
-Peter. Mantua collector, 3D printing enthusiast, Korail modeler.
As it happens, I recently bought a copy of David R. Sweetland's "New York Central Steam in Color" (Morning Sun Books). I found several pics of a K11, in all it appeared the smokebox and firebox were the same color as the boiler (black or weathered to a dark gray). However, several pics of K3 Pacifics appear to show them with graphite.
I'd say in general, NYC engines were either all black, or the graphite areas were covered with enough coal dust that they looked pretty black. I'd probably go with all black or 'engine black' (dark gray) for a K11. If you later find pics of that particular engine with lighter smokebox and firebox, you could always add that with paint or even gray weathering powder.
BTW the smokebox and firebox got too hot for paint - it just burned off - so railroads used a mix of graphite and oil on those parts. I believe at some point (maybe 1920s-30s) paint was developed that could withstand the heat, so in later years you could see all-black engines.
A while ago, I bought an old Bowser NYC K-11 with a super botched paint job among other issues. I'm trying to figure out how to paint it, but I have only found one color photo of a K-11. While the boiler is black for sure, the smokebox is more uncertain. Is it black or graphite grey? I see some NYC steam models with black smoke boxes and some with grey. Are they both correct? The one color K-11 photo shows a black smokebox, but it could be colorized.