int boja = 255;
int smer = 1;

void setup() {
  size(160, 160);
  frameRate(120);
  ellipseMode(RADIUS);
  smooth();
  strokeWeight(4);
}

void draw() {
  background(255 - boja);
  noFill();
  stroke(color(boja));
  ellipse(80, 80, 60, 60);//rect(30, 30, 100, 100);
  if ((boja > 0) && (boja < 255)) boja += smer;
  else if (boja == 255) { boja = 254; smer = -1; }
  else if (boja == 0) { boja = 1; smer = 1; }
}