Just don't let the diameter at the mouth get below 0.373". SAAMI spec is 0.3730"-0.3800" at the mouth. The minimum diameter is to ensure the mouth stops on the shoulder at the end of the case portion of the chamber. Too narrow can slip into the throat, trapping the mouth and preventing proper bullet release, which can raise pressure significantly.
Coloring is usually due to a lack of pressure inflating the case hard enough against the chamber to make a seal. As you said in the first post, the NATO loads were way hotter. However, your loads look to be up to 0.2 grains over Hodgdon's maximum for a non-+P load. So I am wondering if maybe you are seeing something caused by the anti-fouling additive used in the CFE powders. It is famous for increasing soot in bores. I don't know if that soot is harder to rub off than non-CFE powders make or not. But if it is sooty enough, it's not impossible that the case is being blackened by expanding to contact soot left in the chamber by the last shot.
Check to see if the first case ejected from a cleaned barrel is blackened or not. If not, then the idea of picking up soot left by lingering smoke and pressure during ejection seems favorable. But if the first case is black, too, then gas is getting back over it before a good seal is established, and that's just a reflection of how fast the powder builds pressure. You could try working the load up over again from about 4.7 grains using magnum primers to see if those get the pressure to build any faster.
A second factor is that the NATO loads are made with the case as-manufactured, which includes a taper outward going back toward the head. When you resize with a die that uses a carbide ring, the ring is just one diameter, so it makes the case straight and thus narrower near the head than a new factory case or one resized in a steel die is. This is why you often can make out the location of the base of the bullet in a reload by looking for the slight bulge around the brass where the bullet stops. But factory ammo doesn't show that bulge due to the taper widening the brass outward at the bullet base.
I edited your photo here to show the side-by-side. You can see the blackening stops where the carbide sizing ring stopped going any further down. That's the part that expands outward during firing.
