Makes the loading screen darker.
Requires OptiFine
You can change the colors by editing Dark_Loading_Screen.zip/assets/minecraft/optifine/color.properties.
If you can't / don't want to use OptiFine, you can try the Fabric mod version of this pack.
Credit to sp614x for putting that feature in OptiFine and to u/Coffee_0304 for making me learn it was a thing. For all of your color-changing needs, check out the OptiFine documentation.