Lerp the alpha value of a color

I want to change the alpha value (transparency) of a color back and forth:

private void Awake()
{
    material =  gameObject.GetComponent<Renderer>().material;   
    oColor   = material.color; // the alpha here is 0
    nColor   = material.color;
    nColor.a = 255f;
}
private void Update()
{
    material.color = Color.Lerp(oColor, nColor, Mathf.PingPong(Time.time, 1));
}

This doesn't work, the color instantly changes to white, and it keeps blinking the original color. I'm following this.