Negative Lights

if Deferred Lighting mode only, you can hack the light's color to be negative. Since the built-in color selector is clamped to normalized colors, you cannot use it. You can set negative colors from code, like, l.color = new Color(-0.4f, -0.4f, -0.4f, 1); or just use the following script and attach it to a light, then hack as you like.

 using UnityEngine;

 [ExecuteInEditMode]
 [RequireComponent(typeof(Light))]
 public class HackedLight : MonoBehaviour {

     public Vector4 hackColor;
     public float multiplier = 1;
 
     void Update ()
     {
         var light = GetComponent<Light>();
         light.color = new Color(hackColor.x, hackColor.y, hackColor.z, hackColor.w) * multiplier;
     }
 }

make a light and the color of the light black, or a dark color.