That is interesting. The illustrations in Ballard's own book about the finding of the Bismarck shows the borders being grey! And this might not seem like such a big deal to a lot of people, but it's such an eye catching feature of the ship that I would like to get it right when I build my model. =)
I found this video from the wreck, look at 3:40, 6:00 and 8:33. I see both grey and red in this video, red more between the planks. Could it be that the borders were red at some point, then the red (and only the red) was painted over with grey at some point, perhaps while in Bergen? (The covering up of the marks when leaving Bergen was apparently done with tarp.)
Would love to get a definitive answer on this one.