How do I set a terrain texture or colour from code?

How do I set a terrain texture or colour (the splatmap) from code to a terrain created from code? This is what I have, I see nothing, where am I going wrong?

 using UnityEngine;
 using System.Collections;
 using Utility;
  
 public class test : MonoBehaviour
 {
     //use this for initialization
     void Start ()
     {
         //create a new terrain data
         TerrainData _terrainData = new TerrainData();
  
         //set terrain width, height, length
         _terrainData.size = new Vector3(20, 1, 20);
  
 SplatPrototype[] terrainTexture = new SplatPrototype[1];
 terrainTexture[0] = new SplatPrototype();
 terrainTexture[0].texture = (Texture2D)Resources.Load("Terrain Assets/Terrain Textures/Grass (Hill)");
 _terrainData.splatPrototypes = terrainTexture;
  
         //Create a terrain with the set terrain data
         GameObject _terrain = Terrain.CreateTerrainGameObject(_terrainData);
     }
    
     // Update is called once per frame
     void Update () {
  
     }
 }