That looks to me like sun exposure. Culms can get a "tan," too!. It's nothing to worry about, as long as you don't mind the permanent color change. I'm guessing that you only see this on the side exposed to the sun, right?

The reddening of the culms isn't permanent, is it? I have aureosulcatas that get red, but it fades.

You might want to consider pulling your atro culms to the ground and covering with a tarp this winter. That will protect them and give you the best chance at size-up next year.

Culms of some species do develop a reddish cast that occurs naturally -- usually in the first months after they shoot up -- and that fades over time. I've seen it even in plants not exposed to much sun, and it occurs on all sides of the culm. I haven't noticed any reddish coloration with my own Ph. vivax 'H.I.', but I haven't had it for long either. My experience with culms that pick up a "tan" from sun exposure is that it doesn't necessarily fade, but that could vary from species to species, too, I guess, or be location dependent. I don't want to come off as an expert on all species, since I'm not. My main point for the original poster is that, whatever is occurring with that bamboo, it's not a reflection on the health of the bamboo and shouldn't be a cause of worry.

The "tan" is on the west side, so I suppose that could be it! Hopefully in a few years it will be leafy enough to shade itself.

The soil around my Atro was quite poor, so I bought an overpriced bag of dirt and mixed it in with the soil on top, I'll find out next year if that worked.

The tarp idea sounds plausible, but I'd worry it would be an oven under there, the culms from last winter did not die (they did turn yellow which I thought was interesting) so I think either way I'm going to lose my leafs. I'm starting to get suspicious that I don't have an atro at all, but just some sort of screen bamboo..

nonyaz-
I have a number of bamboo that are normally green-culmed, but they turn a yellowish color after sun exposure, even in the fairly overcast area I live in (Pacific NW). Such a color change doesn't mean that you are going to lose the bamboo, or that your Atro is not Atro. I have an Atro that was deep green when I took the division from a friend (where it was growing in a shady grove), and turned yellowish over a year or so from sitting out in the open in my yard. When culms near the end of their normal life (about 7 years for most species) and/or are dying, they start to turn tan or even gray; they also lose their surface glossiness and look dull.

That reddening is from sun exposure and only occurs on new culms of species with gold culms. After hardening off the red disappears.