texture2D opacity?(javascript)

Hello again! So I'm trying to create a crosshair for an FPS but I don't want it covering the center of the screen so I want to change the opacity. But how? I've found C# tutorials on it but I need a javascript solution. It would be great if I could change it outside the code but if I have to write some extra code here's my script just in case:

 #pragma strict
 
 var CrosshairTex : Texture2D; //The texture variable
 var Params: Rect; //The crosshair parameters variable.
 function Update() {
 Params= Rect(375,150,25,25);//Setting the parameters
 }
 
 function OnGUI() {
 GUI.DrawTexture(Params, CrosshairTex); //Drawing the crosshair.
 }

There's my code. please don't give me new ways to do this unless it is absolutely necessary for this purpose. Please help! Thanks!

P.S: Could I please have the method to do this in unity? I'm not very experienced with my 2d graphics software(inkscape).