Instantiate A Prefab With A Random Color

Hi I've been trying to find a way to instantiate a prefab with a random color. I have looked at many questions with similar titles I'm sure they are good answers but they are not what I need. I am making a replica of a game, for practice and I want to have to random colors to be used when instantiating an object here is my code:

private Vector3 spawnPos = new Vector3(0, 0.75f, 0);
[SerializeField]
private GameObject hitCube;
private Color color1;
private Color color2;
private string lastColor;

// Use this for initialization
void Start () {
    color1 = Random.ColorHSV(0f, 1f, 0f, 0f);
    color2 = Random.ColorHSV(0f, 1f, 0f, 0f);
    for (int i = 0; i < 21; i++)
    {
        if (lastColor == "color1")
        {
            hitCube.GetComponent<Renderer>().sharedMaterial.color = color2;
            lastColor = "color2";
        }
        else
        {
            hitCube.GetComponent<Renderer>().sharedMaterial.color = color1;
            lastColor = "color1";
        }
        Instantiate(hitCube, spawnPos, Quaternion.identity);
        spawnPos.y += 0.50f;
    }

}

So does anyone know how I can make this work? When I use this the ground becomes one color and so do all of the cubes.

Another question I have is why does the ground change color but the launcher doesn't (it looks like it does but actually it is already set to be a darker shade of grey), and why does the ground change color in the first place? I understand that

sharedMaterial

is not the way to go because it is for all of the prefabs. All help is appreciated.