function preload() {
  for (var i = 1; i <= possibleImagesCount; i++) {
    imageFiles.push(loadImage('./PNG150/Seite_' + nf(i, 3) + '.png'));
  }
}
Loads images and searches for the most saturated color. It tries to avoid
the special green color that we've chosen for the book. Otherwise most of
the colors would be that green.
Returns an array with these colors and prints them to the console.