Sun, or water temps, would be my first guess. Also, the larger the stream, the richer it usually is. The richer a stream is, the less opportunistic, and the more schedule oriented the fish become. Timing becomes much more important, and you were at high noon on a sunny day. I realize the stream may not have been "rich", but it's not a black white thing, it's a sliding scale with many shades of gray.
But as an alternative possibility, did you have flooding this summer/fall? Those fish may be gone for now, upstream or down, or even into tribs.
You said it was above a reservoir, is it possible the reservoir backed up to that point? As the water slows into a reservoir during floods, you will deposit a load of silt, well above what is typical. The fish will leave, either to the depths of the main lake, or upstream to find more current/less silt. It might take years of no flooding to rescour that silt out and deposit it down into the lake. Brookie streams are not constant things, they always change, sometimes for the better, sometimes for the worse.
Anyway, regarding post colors, for me, white seems to show up a LOT better than pink/red. I dunno why, it doesn't make sense (as was said, the bubbles are white too). I think that's a personal thing, everyone's eyes work a little different.