with timer you already have a solution for color

I added another decreasing var called widthParticle for the size....

1. ArrayList psystems;
2. void setup() {
3.   size(800, 600);
4.   colorMode(RGB, 255, 255, 255, 100);
5.   psystems = new ArrayList();
6.   smooth();
7. }
8. void draw() {
9.   background(0);
10.   for (int i = psystems.size()-1; i >= 0; i--) {
11.     ParticleSystem psys = (ParticleSystem) psystems.get(i);
12.     psys.run();
13.     if (psys.dead()) {
14.       psystems.remove(i);
15.     }
16.   }
17.   psystems.add(new ParticleSystem(int(random(1, 3)), 0.1, new PVector(random(350, 450), 700)));
18. }
19. color[] colors = {
20.   color(0xC94053, 255), color(0x993559, 255), color(0x6B274E, 255), color(0x421F3E, 255), color(0xFFAA68, 255)
21. };
22. // A simple Particle class
23. class Particle {
24.   PVector loc;
25.   PVector vel;
26.   PVector acc;
27.   float r;
28.   float timer;
29.   float widthParticle;
30.   // Particle Constructor
31.   Particle(PVector l) {
32.     acc = new PVector(0, 0, 0);
33.     vel = new PVector(random(-0.05, 0.05), random(-3, 0), 0);
34.     loc = l.get();
35.     r = 10.0;
36.     //durée de vie
37.     timer = 150.0;
38.     widthParticle=90;
39.   }
40.   void run() {
41.     update();
42.     render();
43.   }
44.   // Method to update location
45.   void update() {
46.     vel.add(acc);
47.     loc.add(vel);
48.     timer -= 1.0;
49.     widthParticle-= 0.23;
50.     // println(timer);
51.   }
52.   // Method to display
53.   void render() {
54.     noStroke();
55.     fill(colors[(int)random(colors.length)], timer);
56.     //
57.     if (widthParticle>0) {
58.       triangle(loc.x, loc.y,
59.       loc.x+widthParticle/2, loc.y-widthParticle,
60.       loc.x+widthParticle, loc.y  );
61.     }
62.     //
63.   }
64.   //
65.   // Is the particle still useful?
66.   boolean dead() {
67.     if (timer <= 0.0) {
68.       return true;
69.     }
70.     else {
71.       return false;
72.     }
73.   }
74. }
75. // An ArrayList is used to manage the list of Particles
76. class ParticleSystem {
77.   ArrayList particles;    // An arraylist for all the particles
78.   PVector origin;        // An origin point for where particles are birthed
79.   ParticleSystem(int pNum, float pChance, PVector v) {
80.     particles = new ArrayList();              // Initialize the arraylist
81.     origin = v.get();         // Store the origin point
82.     for (int i = 0; i < pNum; i++) {
83.       if (random(1) < pChance) {
84.         particles.add(new Particle(origin));
85.       }
86.     }
87.   }
88.   void run() {
89.     // Cycle through the ArrayList backwards b/c we are deleting
90.     for (int i = particles.size()-1; i >= 0; i--) {
91.       Particle p = (Particle) particles.get(i);
92.       p.run();
93.       if (p.dead()) {
94.         particles.remove(i);
95.       }
96.     }
97.   }
98.   // A method to test if the particle system still has particles
99.   boolean dead() {
100.     if (particles.isEmpty()) {
101.       return true;
102.     }
103.     else {
104.       return false;
105.     }
106.   }
107. }

wow, thanks Chris,