createCanvas(500, 500);

  background(150);

} 

​

function draw() { 

  // for(var xPosition = 0; xPosition <= width; xPosition += 50) {

  //   var colorValue = map(xPosition, 0, 500, 0, 255);

  //   fill(colorValue, 100, 100);

  //   // fill(xPosition, 100, 100);

  //   ellipse(xPosition, height/2, 50, 50);

  // }

  var ellipseColor = map(mouseX, 0, 500, 0, 150);

  fill(ellipseColor);

  ellipse(mouseX, mouseY, 40, 40);

}