How do I lerp to a color?

Put your sprite in the scene, add collider to it and the script below. You can use OnMouseEnter simply turning it into coroutine (or you can create a separate coroutine and run it from "normal" OnMouseEnter). Here's the code:

     private SpriteRenderer sr;
     
     private void Awake()
     {
         sr = GetComponent<SpriteRenderer>();
         sr.color = new Color(1f, 1f, 1f, 0f);
     }
     
     IEnumerator OnMouseEnter()
     {
         Color start = sr.color;
         Color target = new Color(1f, 1f, 1f, 1f);
         float t = 0f;
         float duration = 3f;
         while(t < 1f)
         {
             t += Time.deltaTime / duration;
             sr.color = Color.Lerp(start, target, t);
             yield return null;
         }
     }