GUI.DrawTexture with opacity

You can also alter the GUI.color (specifically GUI.color.a) right before the call to GUI.DrawTexture() to determine the transparency. Just make sure to return it to the previous color after it if you draw anything else. You can even make the entire GUI fade away, buttons and all...

This works just fine:

 using UnityEngine;
 using System.Collections;
 
 public class GUITest : MonoBehaviour 
 {
     public Texture2D     txForeground;
     public float        fAlpha;
     
     void OnGUI()
     {
         // Unless you changed it, the default for GUI.color is Color.white, but better be safe than sorry. 
         // Just save the previous color and restore it after you draw whatever you want to be transparent.
         Color colPreviousGUIColor = GUI.color;
         
         GUI.color = new Color(colPreviousGUIColor.r, colPreviousGUIColor.g, colPreviousGUIColor.b, fAlpha);
         GUI.DrawTexture(new Rect(0,0,Screen.width, Screen.height), txForeground);
         
         GUI.color = colPreviousGUIColor;
     }
 }

You can specify the the opacity of a texture by setting each pixel's alpha value. However this is quite slow for fading animations (requires many SetPixel calls), so you'd better use a regular Plane (GameObject > Create other > Plane) in front of the camera showing the texture. You can change the transparency by setting the Alpha value for the base color (ensure to use a transparent material).

ОК, I'll try it. Or maybe Graphics.DrawTexture can help? It has Material and Color in parameters list. So maybe I can to change alpha channel using it?

Very good idea. Do this function change all GUI elements? I have 6 textures on the screen and each of them must have different opacity level.

Oh, I understand how it works. Thanks to all for help!