Stuart, it just can't be said that shop guys painted engines however they wanted. There were standards and specifications which were expected and conformed to on all major lines.
Black was certainly specified for reasons of cost and practicality.
While some shops, like Salida, had more time to add certain detail work on engines being painted, they were still expected to conform to company standards, which were very specific about color and even formulas of different types of paint used and where it was to be used. Some was specified to meet higher heat requirements, other paint was specified for reasons of economy.
You can bet that everything that happened on any major railroad was done for a specific reason, and economy was always a consideration.
I'm not sure if this helps answer your question, but this was definitely not an area that was open to varied expression by various shops.