That is an interesting question. I know that in the early days, engines were very ornate, and then approaching the turn of the century, utilitarianism took over. Black seems like a very practical color for coal burning locomotives. It also seems to look right, especially with some small accents of red, brass, etc. I think Caterpillar yellow looks great on earthmoving equipment. Can you imagine a Cat yellow steam locomotive?
But this raises another question that is similar. Why was so much rolling stock painted Tuscan red? Coal soot made locomotives naturally black. Did rust make rolling stock naturally red oxide looking? Why wouldn’t you paint boxcars black?
I have always heard that the reason most farmers painted their barns red is because red paint was the cheapest, and the reason it was the cheapest was because they sold so much of it for barns. I like the black barns you see in Kentucky for instance.
RK