Changing colour of shapes over time

Hi everyone,

Now I'm very rusty with processing so I'm 100% sure I'm making some dumb mistakes here.

I'm trying to make the ellipses in this sketch change colour as they become older but I can't figure it out for the life of me!

Thanks in advance for any help/suggestions :)

See code here:

import ddf.minim.*;

Minim minim;
AudioInput in;
 float volume = 0;
float volumeF = 0;

   int maxParticles=1000; // max number of particles. real particles count depends on ppf and its lifetime
int addPPF=2;         // number of particles per frame added
ArrayList particles;

void setup(){
  size(900,900, P3D);
  frameRate(30);
  smooth();
  background(0);
  noStroke();
     minim = new Minim(this);
  minim.debugOn();

  // get a line in from Minim, default bit depth is 16
  in = minim.getLineIn(Minim.MONO, 512);


  particles = new ArrayList();
}

void draw(){
  background(0); // overpaint circles the frame before
  //  for(int i = 0; i < 1; i++)
  //{
     volumeF = in.right.level()*1000;
  volume = 0.8*volume + 0.2*volumeF;
  if(particles.size()<maxParticles){
    // add particle(s)
    for(int k=1; k<= volume; k++){
      particles.add(new Particles());
    }
  }
  //}

  // run trought all Balls and make some action
  for(int j=0; j<particles.size(); j++){
    Particles particle = (Particles)particles.get(j);
    // if particle still alive
    if( particle.alive() ){
      // update x/y positions
      particle.position();
      // now draw as cirle
      if( particle.lt < 200 ) fill(20, 20, 20, particle.lt);
      else if(particle.lt >= 200) fill(50, 50, 50, particle.lt);
      else if(particle.lt >= 300) fill(100, 100, 100, particle.lt);
      else if(particle.lt >= 400) fill(150, 150, 150, particle.lt);
      else if(particle.lt >= 500) fill(200, 200, 200, particle.lt);
      else if(particle.lt >= 600) fill(250, 250, 250, particle.lt);
      //stroke(random(100));
      ellipse(particle.x, particle.y, particle.r, particle.r);

  }else{
      // if lifetime is running out, delete
      particles.remove(j);
    }
  }
}


  void stop()
{
  // always close Minim audio classes when you are done with them
  in.close();
  minim.stop();

  super.stop();
}

public class Particles {
  float r, x, y, _x, _y, dx=0, dy=0, lt;

  // init start-values
  public Particles() {
    // start position center
    x=width/2;
    y=height/2;
    // now random values to make it non-linear
    lt=random(5,2000);       // lifetime in frames
    r=random(1, 4);        // circle Radius
    _x=random(-0.001, 0.001);    // stepwidth in x-pos (left, right)
    _y=random(-0.001, 0.001);    // stepwidth in y-pos (up, down)
  }

  public void position() {
    // stepwidth increases, circlespeed is exponential
    dx+=_x;
    dy+=_y;
    x+=dx;
    y+=dy;
  }

  public boolean alive(){
     if(lt<=0){ return false; }else{ lt--; return true; }
  }
}