var b = 255;

​

function setup() {

  createCanvas(1024, 768);

​

}

​

function draw() {

​

  // background

  b = map (mouseX,0,1024,0,255);

  g = map (mouseX,0,1024,255,0);

  background(0,g,b);

  // ellipse

  fill ( 255,0 ,255);

  ellipse(mouseX, mouseY, 10, 10);

​

​

}