I narrowed down the following sky problem to astral sorcery + optifine + forge with shaders. I loaded just the two mods and recreated the issue in the image below. When I removed astral sorcery the sky returned to normal. To be completely fair to Astral Sorcery, when I removed Optifine, the sky looked fine as well just without all the pretty. This is with Minecraft 1.12.2 using astralsorcery-1.12.2-1.8.2, Optfine preview_OptiFine_1.12.2_HD_U_C7_pre.jar(It did the same thing on previous stable versions of Optifine), and Forge 1.12.2-forge1.12.2-14.23.0.2549 using just about any shader pack, but I was using SEUS-v11.0.zip for the image.

I don't support shaders. There's a config option 'weakSkyRenders' where you can add the dimension ID's AS should not do its skyrender in. Add whatever dimension you need it for there and it'll - well - disable AS' skyrender there.

How do you enter configurations? I am having this same issue. Can we get a written guide on how to do this I keep seeing the same answer but no one really says how.

It's in the config file. Try reading it. Plug in the dimension id, which is what is said in the config, and by Hellfire up above.