Fade in/out continously

Hello!

I make a scene that it's seperated in 4 parts. How can I make the first part to start with a fading from black, and right before second part to fade out to a black sreen and then again to fade in?

Sorry for my english!

This is the script that I use, but it only makes the screen to fade in from black on start.

     #pragma strict
      
     // FadeInOut
      
     var fadeTexture : Texture2D;
     var fadeSpeed = 0.2;
     var drawDepth = -1000;
      
     private var alpha = 1.0;
     private var fadeDir = -1;
      
     function OnGUI(){
      
     alpha += fadeDir * fadeSpeed * Time.deltaTime;
     alpha = Mathf.Clamp01(alpha);
      
     GUI.color.a = alpha;
      
     GUI.depth = drawDepth;
      
     GUI.DrawTexture(Rect(0, 0, Screen.width, Screen.height), fadeTexture);
     }