Default. The rest of the weapon skins just don't seem "neutral" enough to go with any armor color combo.
I feel like this is something you have to either embrace or give up on. Most of the time I give up on having my weapon skins match, or not outright clash with, my armor (and I don't think it's possible to make an outfit that the Pop Culture skins don't clash with).
But other times, I deliberately make my armor match my weapon skins as closely as possible, which can sometimes have surprisingly satisfying results. I invite you to try Venator Retiarus colored pumpkin+pepper with a Dynasty visor, and see how nice those Aurum weapon skins look when paired with that outfit. That's just one example of what I like to do. But, for the most part, I think you're right. Default is probably as neutral as the weapons can get. Gold Standard might be close, though.
I use Landgrave on everything because gold + full set. I would use the red/white/blue HCS skins if they had dmr/smg variants too, but alas they do not.
Yeah, I wish they were a full set. It's annoying to have to mix and match because something isn't a full set.