Just to clear up the firebox issue… Locomotives burning anthracite as a fuel require a larger grate area because of the slow burning properties of anthracite. (Actually, most locomotives burned a low-grade type of anthracite known as culm.) The Reading Company designed the Wooten firebox, which is very wide, to provide this increased grate area. The width of the firebox caused problems in mounting the locomotive cab to the rear of the firebox, so the cab was placed forward of the firebox, straddling the boiler, to create the camelback. Camelbacks were used extensively on the anthracite roads, which included the Reading, the Jersey Central, the Lackawanna, and the Lehigh Valley. Later locomotive designs solved the problem of mounting the cab to the rear of the Wooten firebox. I don’t believe that any camelbacks were built after the 1920’s.
On smokeboxes and firebox sides which were not lagged (insulated) - that is, most of them - paint would generally burn off quickly, exposing the steel to the weather, with the resulting rust. To combat this, the smokeboxes were coated/painted with a mixture of powdered graphite and heavy oil. The primary reason for differences in shade when first applied was that the mixture was generally not very precise, so whenever the supply in the roundhouse got low, the workers would throw into the pot a scoop of graphite and/or oil, whichever seemed appropriate at the time. As the oil evaporated, the color would turn more of a silvery grey, with the graphite protecting the hot surfaces even when the oil was completely dried up.
Some railroads liked the silvery look (the Pennsy comes to mind), so they were more careful about the mix, especially on the passenger engines. In later years, as paint technology developed, heat-resistant paint enabled other colors to be applied - aluminum, gloss black, etc.