I custom paint musical instruments for people. I do charge. It is a very, very side business that began with me customizing my own. As others have said in this thread, I go over fully what the person wants right down to color and finish type (flat, gloss, metallic, anodized looking, etc..)
I do send pictures or have them over as I proceed with each step. It's not a volume business, and not a big moneymaker, but it does put some extra cash in my pockets. They key is to be very clear and explain all the steps.
I say go for it. Your example here are phenomenal. If I needed weathered trains I know I would reach out to you.
While not a train, here's an example of what I do.
It's a new pro portable keyboar that someone wanted to pop with an anodized aluminum look. The top shell was simple black hard plastic. I use Duplicolor Automotive paints
It's in a box in the attic, but I do have an old diesel loco that I got crazy with and refinished with Duplicolor. I'll go find it and post a pic. It's a dark purple "Plum Crazy". Why? Because I can