createCanvas(400, 400);

  noStroke();

  background(220);

} 

​

function draw() {

  var xColor = map(mouseX, 0, width, 0, 255);

  var yColor = map(mouseY, 0, height, 0, 255);

  fill(xColor, yColor, 200);

  ellipse(mouseX, mouseY, 50, 50);

}