The Feb 1978 "Trains" had a quite definitive article on the TP 2-10-4's by Charles M. Mizell, Jr. The article addresses the color issue:
"Quite a few of us saw the 600's, but evidently very few of us looked at them. There is more disagreement about the color of T&P locomotives than any other point. As best as I have been able to determine, they were gloss black, with "Sta-brite" silver graphite smokeboxes and fireboxes; red oxide cab roofs between the eaves; brass safety valves, whistles and bells; burnished rods and valve gear. Freight engines had black stack rims and valve and cylinder head covers, passenger engines had nickel silver. Lettering was gold. And yet, builder specifications required that boiler jacket, air pumps and cylinder covers be painted in Charles R. Long Co. no. 162 "Light Russia".
Mr. Mizell goes on to say that Russian Iron alloy was used in the 19th century to deter rusting, but the color was was carried forward in paint, varying from a greenish tint to a definite gray. "One old painter said we mixed it up until we got the right color, and that's what we used". Mr. Mizell goes on to say, "however, I remember seeing the 616 about 1947 and remarking to myself that it was black". My bet is that locomotives came out of shops after getting classified repairs, painted in #162 "Light Russia", but after time on the road and getting repainted, they ended up in black. Hence, either black or Russia Iron would be correct for these locomotives. We all can rest easy !!