So....the yellow appears to be coming back a bit. At first I thought it was my imagination, because it's very, very minor. It just looked a bit...dingy. But to test, we rubbed a vitamin C tablet on the grout in the skimmer (where our staining showed up first, and worst, last time) and it turned back to bright white.
What could cause this? We JUST got our chlorine and Ph back to normal. We used sequestering agent following the ascorbic acid treatment, and put a CuLator packet in the skimmer as well. Are there some metals that sequestering agent doesn't work with? Our packet hasn't changed color yet so I don't even know for sure what kind of staining we are getting. I'd post a pic but it's so minor right now you'd never see it.