The big boy I saw at Steamtown is black, can't say I've ever seen photos that looked grey, like some of the Challengers, but that of course doesn't mean there weren't any.
Gary...The Big Boy did have a silver-grey smokebox and fire box when Steamtown was in Vermont. I built the Revell static model to that same one. I'll see if I have a pic of it. I'm sure I do somewhere.
Also, My Rivorossi Big Boy is decoed the same way.
Get some Polly Scale ATSF silver and mix in some black until it looks right to you. Brush paint a little on a scrap piece and see how it looks once it's dry then make adjustments from there.