size(400,400);
background(255,255,255);
ellipseMode(CENTER_DIAMETER);
smooth();
noStroke();

fill(50,50,50,150);
ellipse(50,50,50,50);

fill(100,100,100,100);
ellipse(100,100,100,100);

fill(150,150,150,50);
ellipse(150,150,150,150);