Change colour of particles with C#

Hello

I have a Particle System with particles that have an infinite lifetime. But i want to make these particles change colour, or rather, change their alpha over time using a random value.

I appled this code, but get this error:

 NullReferenceException: Object reference not set to an instance of an object
 Flicker.Update ()
 

Code:

 using UnityEngine;
 using System.Collections;
 
 public class Flicker : MonoBehaviour {
 
     ParticleSystem particleSystem;
     ParticleSystem.Particle[] particles;
 
     private int totalParticles;
     private Color32 color;
     void Start(){
         particleSystem         = GetComponent<ParticleSystem>();
         totalParticles         = particleSystem.particleCount;
     }
 
     void Update(){
 
     for(int i = 0; i < totalParticles; i++){
         color = new Color(1,1,1,Random.Range(0.8f,1));
         particles[i].startColor = color;
     }
 
         particleSystem.SetParticles(particles,totalParticles);
     }
 }

What am i getting wrong here?