Highlight a part of a chess table

I would use an array of arrays to create and store information about your squares and to reference them. then you just use your x and y coordinates to reference them.

     public float timer;
     GameObject[][] squares;
     void Start () {
         int x = 8;
         bool col = false;
         squares = new GameObject[x][];
 
 
         while (x>0) {x--;
             col =! col;
             int y = 8;
             squares[x] = new GameObject[y];
             while(y>0){y--;    col =! col;
                 squares[x][y] = GameObject.CreatePrimitive(PrimitiveType.Cube);
                 if(col){squares[x][y].renderer.material.color = Color.black;}else{
                     squares[x][y].renderer.material.color = Color.white;}squares[x][y].transform.position = new Vector3(x,y,0);
                 }}
         Camera.main.transform.position = new Vector3 (4, 4, 5);
         Camera.main.transform.eulerAngles = new Vector3 (0, 180, 0);
         Camera.main.orthographicSize = 10;
     }
     
 
     void Update () {
         timer -= Time.deltaTime;
         if (timer < 0) {
                         timer += 1;
             highlight(Random.Range(0,8),Random.Range(0,8));
                 }
     
     }
     public void highlight(int x, int y){
         squares [x] [y].renderer.material.color = Color.green;
 
     }