var g;

var b;

​

function setup() {

  createCanvas(windowWidth, windowHeight);

  background(0);

}

​

function draw() {

  r = map(mouseY, 0, windowWidth, 0, 255);

  g = map(mouseX, 0, windowHeight, 0, 255);

  b = map(mouseY, 0, windowWidth, 255, 0);

  background(r, g, b);

    fill(0, 0, 0, 0.2);

    ellipse(mouseX,mouseY,20,20);

}