Reported by one of my visitors, that NPCs that are not attackable should not display their /con color when targetted... but instead should show a white reticle.

Are you sure the particular NPC is not set to attackable? There is already a check that should prevent this.

I will double check to make sure.

You are correct. My problem was a lack of /repop'ing the spawn after changing it's attackable to 0. Once repopped, the NPCs do display the white reticle, so this is a closed issue.
Thanks for pointing that out.