Seems like you want to edit the color of the sprite renderer renderer. Get the renderer using GameObject.GetComponent(). From there change the color of the material using SpriteRenderer.color. Since you want your object to get bluer you probably want to start with pure white and go to pure blue, i.e. Color(health/100, health/100, 1, 1) would change the color linearly from white (1, 1, 1) at 100 health to blue (0, 0, 1) at 0 health.
void OnCollisionEnter2D (Collision2D coll)
{
if(coll.gameObject.tag == "Rain")
{
//Destroy (coll.gameObject);
Health -= 10;
gameObject.GetComponent<SpriteRenderer>().color=new Color(health/100f, health/100f, 1, 1);
}
if (Health == 0)
{
Destroy(gameObject);
}
}