size(600,500);

background(0);

noStroke();

// trick to first cover background with opaque yellow
fill(255,255,0);
ellipse(400,250,300,300);

// now opqaue red color over that
fill(212,0,44);
ellipse(200,250,300,300);

// and then finally transparent yellow on top of the red
// (and on top of the existing yellow)
fill(255,255,0,127);
ellipse(400,250,300,300);  // blend with red