So you are going to be getting values here of 0-5 which is beyond the size if you array. 

 

2. 
   
3. Circle[] circles;
4. 
   
5. 
   
6. int numCircles = 13; 
7. int maxDistance;  //max distance between circles when draw lines
8. int m;
9. 
   
10. float bgX;
11. float bgY;
12. 
    
13. float randomVal;
14. float circleX;
15. float circleY;
16. 
    
17. PImage bg;
18. PImage bgRed;
19. 
    
20. color[] colors;
21. int indexColors;
22. float opa;
23. 
    
24. 
    
25. 
    
26. void setup() {
27.   size(displayWidth, displayHeight);  //size of window
28.   smooth();  //draw circles with anti-aliased edge
29.   m = millis();
30.   
31.   bgX = (displayWidth-800)/2;
32.   bgY = (displayHeight-800)/2;
33.   
34.   //frameRate(15);
35.   
36.   bg = loadImage("bg_trans.png");
37.   bgRed = loadImage("bg_red.png");
38.   
39.   float opa = random(100, 255);
40.   
41.   colors = new color[5];
42.   
43.   colors[0] = color(255, 206, 0, opa); //yellow
44.   colors[1] = color(255, opa); // white
45.   colors[2] = color(0, 58, 142, opa); //blue
46.   colors[3] = color(226, 0, 0, opa); //red
47.   colors[4] = color(73,73,73, opa); //black(gray)
48.   
49.   indexColors = int(random(colors.length));
50.   
51. 
    
52.   // create an array and fill it with circles
53.   circles = new Circle[numCircles];  //create an array of circles
54.   for (int i=0; i< numCircles; i++) {
55.     float randomAngle = random(0, TWO_PI);
56.     float randomRadius = random(20, 350); // Not too close of the center, adjust at will
57.     float circleX = width/2 + cos(randomAngle) * randomRadius;
58.     float circleY = height/2 + sin(randomAngle) * randomRadius;
59.   
60.     println(circleX + ", " + circleY);
61.   
62.     //random position in the window, random size between 5-15
63.     circles[i] = new Circle(circleX, circleY, random(10, 30), colors[indexColors]);
64.   }
65. }
66. 
    
67. void draw() {
68.   // clear background
69.   background(0,0,75);
70.   tint(255, 50);
71.   image(bg, bgX, bgY);
72.   
73.  // tint(255, 50);
74.  // image(bgRed, bgX, bgY);
75. 
    
76.   
77.   for (int i=0; i< numCircles; i++) {
78.     circles[i].update(); 
79.   }
80. 
    
81.   // define maximum distance
82.   maxDistance = 180;
83.   
84.   //maxDistance =mouseX;  //manuplate lines by mouse cursor
85.   
86.   // look of the lines
87.   stroke(255, 130);
88.   strokeWeight(0.5);
89. 
    
90.   for (int i=0; i< numCircles; i++) {
91.     // compare circle to other circles
92.     for (int j=i+1; j< numCircles; j++) {
93.       // draw line if distance is below 'maxDistance'
94.       if (dist(circles[i].x, circles[i].y, circles[j].x, circles[j].y) < maxDistance){
95.         line(circles[i].x, circles[i].y, circles[j].x, circles[j].y);
96.       }
97.     }
98.   }
99.     // update and display the circles
100.   for (int i=0; i< numCircles; i++) {
101.     circles[i].display();
102.   }
103. }
104. 
     
105. void mousePressed() {
106.   setup();
107. }
108. 
     

2. class Circle {
3.   float x, y, dia;
4.   color col;
5.   
6.   float speed=0;
7.   float gravity=0.2;
8.   
9.   float fallLimit = bgY + 780;  //limit y for falling stars
10.   
11.   
12.   Circle(float x, float y, float dia, color col) {
13.     this.x =x;
14.     this.y =y;
15.     this.dia = dia;
16.     this.col = col;
17.   }
18.   
19.   
20. //circles move slightly in their position
21.   void update() {
22.     // code for movement here
23.     // this is just some random displacement
24.     x = x + random(-0.5, 0.5);
25.     y = y + random(-0.5, 0.5);
26.     
27.     int m = millis();
28. 
    
29.     speed = speed + gravity;
30.     
31.     
32.     if(y < fallLimit){
33.       y = y + speed;
34.     }
35.     if (y > fallLimit) {
36.       y = fallLimit;
37.     }
38.     
39.     
40.     /*
41.     if( get(x,y) == color(255,0,0) ){
42.       y = y + speed;
43.     }
44.     if( get(x,y) == color(0,0,255) ){
45.       y=y;
46.     }
47.     */
48.   }
49. 
    
50. 
    
51.   
52.   void display() {
53.     // code for drawing the circles
54.     noStroke();
55.     fill(col);
56.     ellipse(x, y, dia, dia);
57.   }
58. }