How to Event tigger when drag and drop complete

If you use Unity UI and an Eventsystem you could have a script on each item that implements IDropHandler and IDragHandler. Here is an example that simply drag and drop a colour.

 using UnityEngine;
 using UnityEngine.EventSystems;
 using UnityEngine.UI;
 
 public class MyDrag : MonoBehaviour, IDropHandler, IDragHandler
 {
     Image myImage;
 
     void Awake()
     {
         myImage = GetComponent<Image>();
     }
 
     public void OnDrop(PointerEventData eventData)
     {
         if (eventData.pointerDrag != null)
         {
             myImage.color = eventData.pointerDrag.GetComponent<Image>().color;
         }
     }
 
     public void OnDrag(PointerEventData eventData)
     {
     }
 }