createCanvas(600, 400);

  background(100, 0, 100, 100);

  rectMode(CENTER);

  // fill(255, 0, 0);

  // noStroke();

  // rect(width/2, height/2, 50, 50);

  // stroke(1);

  // line(0, 0, width, height);

  // line(0, height, width, 0);

  noStroke();

} 

​

function draw() { 

  //mouseX and mouseY

  var xColor = map(mouseX, 0, width, 0, 255);

  var yColor = map(mouseY, 0, height, 0, 255);

  fill(xColor, yColor, 200);

    rect(mouseX, mouseY, 100, 100);

}