// change the color of the texture to green
guiTexture.color = Color.green;

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    void Example() {
        guiTexture.color = Color.green;
    }
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    def Example():
        guiTexture.color = Color.green