Specifically, my pool was resurfaced with pebble sheen (Riverbend Sandler's version of it here in Dallas) a little over a year ago. I don't remember the color name but it was a blue-colored plaster between the pebbles for an overall bluish look. I did a partial drain and fill (got it about half way down)- at most there was exposed surface for no more than 36 hours (and during daylight, it was mostly cloudy, so I wasn't getting intense sun). I noticed that the surface looked a little more "gray" when exposed; I continued to notice it after the refill, though I didn't pay attention as I was in the middle of fighting some awfully cloudy/dirty water (soil runoff due to horrendous rains). This was a few weeks ago. I'm now noticing that gray tint continue to stay around, and it ends right where the waterline got down to.
PB told us the surface would get bluer over time. So possibly lack of water and direct sun exposure set me back a bit. It took a few days to get chemistry back after the fill but it was never far off (pH no less than 7.2, CH no less than 50-100 for a week, and Alk no less than 40-50 for less than 2 days).
Not sure what else would be going on here besides a little temporary drying out? Scrubbing did nothing for it so there's no layer of something or other that's discoloring it.