Good afternoon Carmen,
I couldn't tell you if this is a Navy thing or not. I sailed for Wijsmuller and Smit-Lloyd on deep sea salvage tugs and anchor handling tugs and we didn't have huge crews that had to be kept busy painting anything that didn't move
The capstans on board the vessels I sailed on were generally painted black with a white segment or stripe on the top. Why did we do this? It made it easier to see if the capstan was still rotating or not. I would guess that the capstans you refer to had stripes painted on them for the same reason. There are plenty of Navy guys on this forum who will undoubtedly now add their 2 cents

and tell us if I am right or wrong.
cheers,
Julian