I see what you mean about the color of those beers. With those grain bills (except maybe the one that used C20), I am a bit surprised that you are getting such a [relatively] dark color. For that reason, I really don't think the issue is with the grain bill per se, but likely some other variable in the brewing process. pH, perhaps? Or as someone else suggested, boiling too hot (resulting in speeding up the Maillard reactions/kettle caramelization)? Not wanting to assume anything, I'll also ask, do you do full volume boils, or do you boil down below your target batch size and top up with water (which would give you a darker color)?
I just have a hard time imagining that a 100% base malt grain bill would be so consistently giving you those darker colors. The only thing I can think of as far as that goes is that your quantities are higher than what I think of as "average strength" (not session) grain bills. Perhaps just the sheer quantity of grain is darkening up your beers...?
I'm just spitballing at the moment, though. Maybe someone else noticed some detail I didn't, or has some better insight.