I want to change the sky color and if possible the sun/moon texture if the player is wearing a type of glasses.

 

But since I'm a beginner in minecraft modding I don't know where to start looking after creating my item.

You can change fog color

@SideOnly(Side.CLIENT)
    @SubscribeEvent
    public void onFogColorRender(EntityViewRenderEvent.FogColors event){
        if(true) {
            event.setRed(200);
            event.setGreen(10);
            event.setBlue(10);
        }
    }

And register it as normal event