Change terrain texture colour

I have two textures for my terrain. As a default I use the green texture. However I want my user to be able to change the settings of the game to make the texture red. I have buttons in place that change a string to either say "Green" or "Red". At the start of my game, the below script is run.

 using UnityEngine;
 using System.Collections;

 public class TerrainTexture : MonoBehaviour {

     public Texture green;
       public Texture red;

     public GameObject mapcheck;

     public Terrain terrain;

       void Update ()
       {
            if (mapcheck.GetComponent<Script>().map == "Green")
            {
             // Switch to Green
            }
            else if (mapcheck.GetComponent<Script>().map == "Red")
            {
             // Switch to Red
            }
        }
 }

I have done a lot of research but I had various errors when I tried to put anything at my comments. What can I put in place to change the colour of the terrain? (Essentially switching the first texture to the second or vice versa).