void setup() {

  colorMode(HSB, 255);
  size(400, 400);
  smooth();
  frameRate(30);
  background(0);
}

void draw() {

  color rc = (color) random(0400);  // random color
  float rx = random (width);        // random x-position of ellipse
  float ry = random (height);       // random y-position of ellipse
  float d  = random (10, 120);      // width & height of ellipse

  //for (int t = 100; t >= 0; t -= 5)   kreis(rc, rx, ry, d, t);

  int t = 0100;
  kreis(rc, rx, ry, d, t);
}

void kreis (color rc, float rx, float ry, float d, float t) {

  noStroke();
  fill(rc, rc, rc, t);
  ellipse(rx, ry, d, d);

  noFill();
  stroke(rc, rc, rc, t);
  ellipse(rx, ry, 4+d, 4+d);
}

color rc = (color) random(0400); 

??

Other question: Why did you change: ...

1.   Ball Balls[] = new Ball[20];
   
   void setup() {
   
    
     size(400, 400);
     smooth();
     background(0);
    
    for (int i = 0; i < Balls.length; i++) {
       Balls[i] = new Ball();
       } 
    }
             
   void draw() {
     background(0);
      
     for (int i = 0; i < Balls.length; i++) {
         
     for (Balls[1].t = 200; Balls[i].t > 0; Balls[i].t = Balls[1].t - i*20 + 20) {
          
       Balls[i].drawShape();
     }
     }
   } 
   

1. class Ball {
     int diameter = 15;
     float x = random (0, width); // x-position of ellipse
     float y = random (0, height); // y-position of ellipse
     color c1 = color (#FF0000);
   
   void drawShape() {
     noStroke();
     fill (c1, t);
     ellipse (x, y, diameter, diameter);
   }
   }
   

/**
 * Opacity Balls (v2.52)
 * by 9egenwind (2013/Feb)
 *
 * http://forum.processing.org/topic/why-doesnt-my-while-loop-work
 */

final static byte numBalls = 20;
final static Ball[] balls  = new Ball[numBalls];

void setup() {
  size(400, 400);
  smooth();
  noLoop();
  noStroke();
  ellipseMode(CENTER);

  for ( byte i = 0; i != numBalls; balls[i] = new Ball(++i * 10) );
}

void draw() {
  background(0);
  for (Ball b: balls)    b.drawShape();
}  

void mousePressed() {
  for (Ball b: balls)    b.reposition();
  redraw();
}

void keyPressed() {
  mousePressed();
}

final class Ball {
  final static short diameter = 60;
  final static color colour = #FF0000;

  final short  opacity;
  short x, y;

  Ball(int opac) {
    opacity = (short) opac;
    reposition();
  }

  void drawShape() {
    fill (colour, opacity);
    ellipse (x, y, diameter, diameter);
  }

  void reposition() {
    x = (short) random(diameter, width  - diameter);
    y = (short) random(diameter, height - diameter);
  }
}