// create a random color, with every color channel between 30 and 230
int red = Greenfoot.getRandomNumber(200) + 30;
int green = Greenfoot.getRandomNumber(200) + 30;
int blue = Greenfoot.getRandomNumber(200) + 30;
int alpha = Greenfoot.getRandomNumber(190) + 60;
img.setColor(new Color(red,green,blue,alpha));
img.fillOval(0, 0, size-1, size-1);
setImage(img);