let b = 255;

let g = 0;

​

function setup() {

  createCanvas(600, 400);

}

​

function draw() {

  r = map(mouseX, 0, 600, 0, 255);

  b = map(mouseX, 0, 600, 255, 0);

  g = map(mouseY, 0, 400, 255, 0);

  background(r,  g,  b);

}