int x,y; 
void setup(){ 
background(0); //la diferencia esta en esta linea 
size(500,500); 
stroke(133,133,133);

} 

void draw(){ 
  
for (int i = 0; i < 100; i++) {
  float r = random(0, 255);

  println(r);
 
x=mouseX-25; 
y=mouseY-25;
fill(255,  r,  r);
rect(x,y,50,50); 
  
}
}