Why did Fat Man have dark paint markings?

You asked a terrific question about the black paint on Fat Man's protruding parts. Fat Man was delivered to Tinian Island in parts and assembled on the island. The implosion design had been tested in New Mexico, but had never been exposed to the kind of humidity it experienced in the Pacific. After assembly, but before loading it onto the bomber Bock's Car,  all of Fat man's seams were sprayed with a sealer. I think it was asphaltum or something similar. Many of the protruding parts were bolted on, so their seams were included in the treatment.