createCanvas(400, 400);

  colorMode(HSB);

  noStroke();

}

​

function draw() {

  let leftHue = map(mouseX, 0, width, 0, 360);

  let rightHue = (leftHue + 180) % 360;

  fill(leftHue, 100, 100);

  rect(0, 0, width/2, height);

  fill(rightHue, 100, 100);

  rect(width/2, 0, width/2, height);

}