Change alpha value of GUI.Box assigned texture?

Im trying to change the alpha value of the texture i have in my GUIStyle that im using on a GUI.Box. Is this possible? Tried this but no luck, wont compile with that syntax, is there a correct form im missing?

var alpha : float;

GUI.Box (Rect (Screen.width-400, 35, 300, 120), "", redWindow.(1, 1, 1, alpha));

Or maybe is there a way to change the alpha through manipulating the GUIStyle? Hm..

Change GUI.color.a before the GUI.Box command then change it back afterward.