Problem with changing the colour of a 3D model on keypress

I'm creating an interactive map and need to change the colours of some of the 3d models on the map. However, the script i am using doesnt change the colour of the model in the game mode but the colour does change in the inspector. I have tried changing the material on the initial model and also tried the unlit setting which was mentioned in another answer. I have a directional light in the scene which is pointing directly towards the model and yet I still cannot get it to work. Could someone please help me?
Here is my code:
using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class ColorChange : MonoBehaviour {

 // Use this for initialization
 void Start () {
 }
 
 // Update is called once per frame
 void Update () {
     if (Input.GetKeyDown (KeyCode.S)) {
         this.GetComponent<Renderer> ().material.SetColor ("_TintColor", Color.red);
     }
 }

}