In the research I have done, I have found that usually they have a maroon-ish cab roofs, and the striping on the walkways is usually present, but faded, and dirty, especially around the air pumps and other places where oil and grime may accumulate. Boilers were usually started life as a glossy black on passenger locos, and matte black on freight, with both fading to a washed out matte grey, almost black over time, and darker graphite for the smoke boxes. as for the wheel pin striping, most of what I have been able to tell shows that passenger locomotives held the striping longer than freights. I didn't get the impression that it was removed, but rather just got dirty, and less prevalent over time. And it seems the passenger locos got a little more attention in the cleanliness department.
As for myself, I have had pretty bad luck with wheel striping. As a matter of fact, recently I replaced all the wheel sets on my GS4 and GS4 based locomotives with new, updated, sets, including my A-4, and I don't plan on repainting the wheels.
I have been using a paint from Games workshops for my black and graphite, its thick and hard to airbrush, but when you can make it work, it produces nice results, even when brushed on. I am at work right now, I'll try to get the color name and see if I have any prototype pictures lying around tomorrow sometime.