createCanvas(400, 400);

  background(220);

  noStroke();

}

​

function draw() {

  const redVal = map(mouseX, 0, width, 0, 255);

  const greenVal = map(mouseY, 0, height, 0, 255);

  fill(redVal, greenVal, 255);

  circle(mouseX, mouseY, 50);

}