PImage pic;
color c;
color cp;

void setup() {
  size(570,353);
  background(100);
 
  // load image
  pic = loadImage("landschaft.jpg");
  image(pic,10,10);
}

void draw() {
}

void mouseClicked() {
  // set cross as cursor type
  cursor(CROSS);
  
  // set previous pixel color to second rectangle
  if (c!=0) {
    cp = c;
    fill(cp);
    noStroke();
    rect(520, 60, 40, 40);
  }
  
  // get color of clicked pixel
  c = pic.get(mouseX, mouseY);
  fill(c);
  noStroke();
  rect(520, 10, 40, 40);
  
}