Question: This may seem like a stupid question, but I have to ask it. In the photo you can see some of the wiring terminations on my new sailboat. Notice the red paint on the studs and hex nut fasteners. What purpose does this red paint serve? My friends and I can’t figure this out.
Answer: This is a good question that actually brings me back to my early days when I was still racing cars. Back then we used to use a brightly colored paint just as you see in your photo, and paint a distinct line on the stud and on one of the flats on the hex nut. (We did a much neater job of it.) In the case of a cap screw or bolt with a hex head, we would paint the fine line on the head and the flat surface adjacent to the head of the bolt after everything had been properly torqued in place. This would allow us to have a quick visual check to see if anything was loosening up while we were racing or immediately following a race. This was an added safety measure to make sure vital components like suspension or steering parts were not vibrating apart in service.
On your boat the use of the paint is a quality-assurance check on the production line for the boat. Once the fastener has been tightened a worker simply puts a dab of paint on the stud to let everyone else on the production line know that the fastener has been tightened at an earlier work station on the production line. Hope this resolves any debate you and your friends are having over this.