Well thanks to this discussion I have confirmed that the mod causing the visual issues in my self-made modpack is NEI.
Yes, in the NEI options I discovered Creative+ button. I pressed it but it didn't change anything. Also, even tho I was in "/gamemode 1" (creative mode), NEI did not let me pull items from the "items screen" and place them in my inventory. Perhaps that is why Creative+ mode was suggested above, but I was unable to figure out how to activate it.
But here was my key moment of discovery. I saw NEI Options has a large button, "NEI Enabled". So I pressed it, changed it to "NEI Disabled", and returned to my game. Guess what? No more visual glitch with the clouds, with the sun, and with the world lighting. Everything was perfectly normal! Re-enabling NEI caused the effect to return.
So I'm getting somewhere. Perhaps the answer is just to remove NEI, but that's a mod I've always felt was kind of manditory for modded, no? Since I'm having a lot of trouble with the mod, perhaps it's worth a try to remove it and see how the game plays without it. Why would just a few of us be having this issue with NEI, but not everybody (since nobody else has chimed in agreeing they have the same problem)? Maybe it's a conflict between NEI and some other mod that not everybody installs?