How to Reset Dark Loading Screen to Default Colors
Deleting the Config File
If you have experimented with colors and want the original look back, close Minecraft completely first. Then navigate to .minecraft/config/ and delete dark-loading-screen.json. On the next launch the mod regenerates the file with its default values, restoring the standard dark loading screen.
If you would rather keep a record of your experiments, rename the file to something like dark-loading-screen.json.bak instead of deleting it. The mod ignores files with other extensions, so it will create a fresh default while your old settings stay on disk.
Resetting Through the GUI
When Mod Menu and Cloth Config are installed, open the mod's config screen and look for a reset button. Cloth Config screens usually provide one that restores every option to its default in a single click. Save and close, then trigger a loading screen to confirm the change.
If colors still look wrong after a reset, another mod may be overriding the loading screen rendering. Disable other loading-screen or splash mods temporarily to isolate the conflict, then re-enable them one at a time.