I would like to change this: "RED(new ColourScheme.Colour(239, 50, 61)"

https://maven.minecraftforge.net/net/minecraftforge/fmlearlydisplay/1.20.1-47.1.0/fmlearlydisplay-1.20.1-47.1.0.jar

...\fmlearlydisplay-1.20.1-47.1.0.jar\net\minecraftforge\fml\earlydisplay\ColourScheme.class

 

// 
// Decompiled by Procyon v0.5.36
// 

package net.minecraftforge.fml.earlydisplay;

public enum ColourScheme
{
    RED(new ColourScheme.Colour(239, 50, 61), new ColourScheme.Colour(255, 255, 255)), 
    BLACK(new ColourScheme.Colour(0, 0, 0), new ColourScheme.Colour(255, 255, 255));
    
    private final ColourScheme.Colour background;
    private final ColourScheme.Colour foreground;
    
    private ColourScheme(final ColourScheme.Colour background, final ColourScheme.Colour foreground) {
        this.background = background;
        this.foreground = foreground;
    }
    
    public ColourScheme.Colour background() {
        return this.background;
    }
    
    public ColourScheme.Colour foreground() {
        return this.foreground;
    }
    
    private static /* synthetic */ ColourScheme[] $values() {
        return new ColourScheme[] { ColourScheme.RED, ColourScheme.BLACK };
    }
    
    static {
        $VALUES = $values();
    }
}

Edited July 23, 20232 yr by omnihil
forgot to specify which loading screen

You don't. Its meant to keep with Mojang's branding colors. If you want a different color use the dark theme option in vanilla, it'll switch to black.

Is there a reason for not adding an option to change it with resource packs, like you can with vanilla loading screen?

You can't change the vanilla loading screen with a resource pack {unless something has changed recently}, for the same reason that you can't change this one. They are done before mods are loaded.