createCanvas(720, 720);

  noCursor();

​

  colorMode(HSB, 360, 100, 100); // hue, saturation, brightness

  rectMode(CENTER); // rectangle grows from centrepoint, not edge

  noStroke(); // no stroke on the rect

}

​

function draw() {

  background(mouseY / 2, 100, 100); // bg fill set by mouseY

​

  fill(360 - mouseY / 2, 100, 100); // square fill set by mouseY

  rect(360, 360, mouseX + 1, mouseX + 1); // square size set by mouseX

}