float fade = 0.0;
color bg = color(random(255),random(255),random(255));
int newR, newG, newB;

void setup() {
  size( $(window).width(),
        $(window).height() );
  background(bg);
  frameRate(30);
}

void draw() {  
  background(bg);
  fade = frameCount % 240;
  if (fade == 0) {
    switchcolor();
  }
  else {
    fade = map(fade, 0, 240, 0, 255);
    fill(newR, newG, newB, fade);
    rect(-10,-10,width+10,height+10);
  }
}

void switchcolor() {
  bg = color(newR, newG, newB);
  background(bg);
  newR = int(random(255));
  newG = int(random(255));
  newB = int(random(255));
}