I just started with Processing 2 (I'm trying to use it to prototype some javascript animation I'm working on) and I can't seem to get my head around blendMode(ADD).  I've reduced my problem to this fairly simple script that I expected to create a simple color wheel, but instead I get unexpected graphical artifacts from the third circle. What am I doing wrong?

1. void setup() {
     size(640, 480);
     background(0);
     noLoop();
   }
   
   void draw() {
     float diam = 300;
     noStroke();
     fill(#ff0000);
     ellipse(320, 160, diam, diam);
     blendMode(ADD);
     fill(#00ff00);
     ellipse(320-diam/3.0, 320, diam, diam);
     fill(#0000ff);
     ellipse(320+diam/3.0, 320, diam, diam);
   }