​

let col = 0;

let r= 0, g = 0, b = 255;

​

function setup() {

  createCanvas(400, 400);

}

​

function draw() {

  //amt of red changes as mouse moves left and right

  r = map(mouseX, 0, 400, 0, 255);

  //we can also invert the range

  b = map(mouseY, 0, 400, 255, 0);

  background(r, 0, b);

  circle(mouseX, mouseY, 24);

}