How blend an UI Image with the background? (Multiply Effect similar to Photoshop)

Hello,

I have an Image , Ui Element, with only a color. Now I want that this this Ui element will blend with my my 2D Level, to get an effect similary to the "Multiply Effect" in Photoshop.

And at the same time I want to be able change the opacity of that Ui Image element. So far i'm only having a script to change the Opacity. I would be happy if somebody could give me a hint how to give this an additional multiply effectas well.

 using UnityEngine;
 using System.Collections;
 using UnityEngine.UI;
 
 public class darkness : MonoBehaviour {
     public float opacity=0.1f;
     Image image;
     void Update() {
         image = GetComponent<Image>();
         Color imageColor = image.color;
 
         imageColor.a = opacity;
         image.color = imageColor;
 
     }
 }