How to reduce gradient banding?

I made a changing gradient background for my mobile game, however there are noticeable banding even though i use 32-bit depth-buffer. How to reduce this?

basically i attach this script to a quad using System.Collections; using System.Collections.Generic; using UnityEngine;

 public class Gradient : MonoBehaviour {
     public Color[] color = new Color[2];
     private static Color[] iColor;
     private static Color[] fColor;
     public float changeColorTime;
     private float timer;
    
     public static Gradient _instance;                
 
     private void Awake()
     {
         if (_instance == null)
         {
             _instance = this;
         }
         else
         {
             Destroy(gameObject);
             return;
         }
         DontDestroyOnLoad(gameObject);
         
     }
 
     private void Start()
     {
         
         
         iColor = new Color[]{ color[0],color[1] };
         fColor = new Color[] { GenerateColor(), GenerateColor() };
         
 
     }
     void Update()
     {
         //Just to make the quad follow the camera
         transform.position = new Vector3(Camera.main.transform.position.x, Camera.main.transform.position.y, transform.position.z);
 
         //change color
         timer += Time.deltaTime;
         if (timer > changeColorTime)
         {
             timer = 0f;
             
             for (int i = 0; i < 2; i++)
             {
                 iColor[i] = fColor[i];
                 fColor[i] = GenerateColor();
             }
         }
 
         _instance.color[0] = Color.Lerp(iColor[0], fColor[0], timer / changeColorTime);
          _instance.color[1] = Color.Lerp(iColor[1], fColor[1], timer / changeColorTime);
 
         MeshFilter thisMeshFilter = GetComponent<MeshFilter>();
         thisMeshFilter.mesh.colors = new Color[] { color[0], Color.black, color[1], Color.black };
     }
 
     Color GenerateColor(float max=0.6f) 
     {
         float r = UnityEngine.Random.Range(0.05f, max);
         float g = UnityEngine.Random.Range(0.05f, max);
         float b = UnityEngine.Random.Range(0.05f, max);
         Color color = new Color(r, g, b);
         return color;
     }
 }
 

and i also use this shader

 Shader "BackgroundPlaneShader" {
                     SubShader {
                        Pass {
                                 ColorMaterial AmbientAndDiffuse 
                        }
                     }
                 }

I hope someone can help me, as for the moment, i will try to learn about shader more. also, do tell me if you also have another solution to make a changing gradient background thanks.

note:I'm completely new to shader