createCanvas(400, 400);

}

​

function draw() {

  var col = map(mouseX, 0, 400, 0, 255);

    background(col);

    var circleColor = map(mouseX, 0, 400, 255, 0);

    fill(circleColor);

    noStroke();

    ellipse(mouseX, mouseY, 50);

}