I just got on here to find this same thing! But I got an answer:

You need to define an array for materials, place the materials in the array by selecting them in the GUI, and then use a function to change the rendering material. Here is my project that involves the object changing material when it's health hits 1:

var health: int; var materials : Material[];

function Update () { if ( health == 1 ) renderer.sharedMaterial = materials[0]; else renderer.sharedMaterial = materials[1];
}

All you have to do is a write a function that controls the array index via mouse click!

Well, here is simple script for GUI button...

var object : GameObject;
var texture : Texture;

function OnMouseDown(){
    object.renderer.material.mainTexture = texture;
}