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HTML 

              
                <!DOCTYPE html>
<html>
<head>
<meta charset=utf-8 />
<title>JS Bin</title>
</head>
<body>
  <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
<title>animazione mostro</title>
<style>
canvas{
	border:0px solid;
}
body{
	background:#CFF;
}
</style>
</head>
  
<body onload="init();">
<canvas width="1200" height="500" id="mycanvas"></canvas>
<script>
  // Michel : I added a global var rectColor here
  var x, y, speed, canvas, context, mousePos, rectColor='red';
  
  
    reqAnimFrame = window.mozRequestAnimationFrame    || 
                window.webkitRequestAnimationFrame ||
                window.msRequestAnimationFrame     ||
                window.oRequestAnimationFrame
                ;
  
  
  function init() {
   
    x =  200;
    y = 150;
    speed = 4;
   
   
    canvas  = document.getElementById("mycanvas");
    context = canvas.getContext("2d");
	

    // Michel : add these listeners here, the canvas will be defined for sure
    // as the init method is called only when the page is loaded
  canvas.addEventListener('mousedown', paint_redrect_yellow);
  canvas.addEventListener('mouseup', paint_redrect_red);
    
    
    canvas.addEventListener('mousemove', function (evt) {
        
        mousePos = getMousePos(canvas, evt);    
        }, false);
		

    animate();
  }
  
  
  // MICHEL : you just need to set the global variable, not set the style or do
  // any drawing. The global variable will be takjen into account by the 
  // drawMonster function
   function paint_redrect_yellow() {
 	rectColor='yellow';
   }
    
	function paint_redrect_red() {
 	  rectColor='red';
    }
     
  
function animate() {
 
  	context.clearRect(0, 0, canvas.width, canvas.height);

 
  drawMonster();
  
  
    x += speed;

    if(x <= 110 || x >= 655){
        speed = -speed; 
    }
    
  
   reqAnimFrame(animate);
}


  
function drawMonster() { 
  
    context.save();  
  	context.translate(x, y);
	context.beginPath()
	context.fillStyle ='rgba(51,51,255,1)';
	context.fillRect(0, 0, 400, 250);	
  
 

  <!--red rectangle -->
    // Michel : use the global variable here
   context.fillStyle =rectColor;
   context.fillRect(25,25,350,200); 
   
  
	
 //right yellow eye
context.beginPath();
context.arc(300,0,70,0, Math.PI/0.5);
context.fillStyle="yellow";
context.fill()
context.strokeStyle = "#3333ff";
context.lineWidth = 5;
context.stroke();

 //right black eye

  /*context.beginPath();
context.arc(280,0,30,0, Math.PI/0.5);
context.fillStyle="#000000";
context.fill()
 */
  //left yellow eye
 context.beginPath();
context.arc(100,0,70,0, Math.PI/0.5);
context.fillStyle="yellow";
context.fill()
context.strokeStyle = "#3333ff";
context.lineWidth = 5;
context.stroke();
  
  
  	
 <!--sopracciglia sn -->
context.beginPath();
context.arc(115,-15,80,0,5, true);
context.strokeStyle="#000000";
context.lineWidth = 13;
context.lineCap ="round";
context.stroke();


<!--sopracciglia dx -->
context.beginPath();
context.arc(290,-20,80,4.2,9.3, true);
context.strokeStyle="#000000";
context.lineWidth = 13;
context.stroke();

  
 <!--mouse first part -->
context.beginPath();
context.moveTo(50,120);
context.quadraticCurveTo(50,150,200,200);
<!--mouse second part -->
context.quadraticCurveTo(350,240,350,150);
context.closePath();

context.save();
context.fillStyle='black';
context.fill();
context.restore();


<!--teeth above-->
context.beginPath()
context.moveTo(180,135);
context.lineTo(190,180);
context.lineTo(210,138);

 context.moveTo(245,142);
context.lineTo(290,180);
context.lineTo(330,150);

<!--teeth below -->
context.moveTo(260,210);
context.lineTo(210,200);
context.lineTo(240,180);

context.fillStyle ="#fff";
context.fill();	

<!--left arm -->
context.beginPath()
context.moveTo(-50,150);
context.lineTo(0,150);

<!--right arm -->
context.moveTo(480,180);
context.lineTo(400,180);

context.lineWidth = 15;
context.strokeStyle = "#3333ff";
context.lineCap ="round";
context.stroke();

<!--tenaglia braccio sn -->
context.beginPath()
context.arc(-80,150,30,0, Math.PI/1.5, true);
<!--tenaglia braccio dx -->
context.moveTo(480,150);
context.lineTo(480,230);
context.lineTo(540,230);
context.lineTo(540,180);


context.lineWidth = 15;
context.stroketyle="#000000";
context.stroke()

context.restore();
  
  

 
  // You told me I have to change x, y + the drawings in the drawEyeThatFollowsMousePos
  // so that the eye has the right colors/position
  // I was only able to change the radius but if i put number instead of x and y the eye does not follow the animation anymore
  //What's wrong with this?
  // Michel ; try x+ something, like the x+150 below
  if(mousePos != undefined) {
     drawEyeThatFollowsMousePos(x+100, y, 50, mousePos.x, mousePos.y)
     drawEyeThatFollowsMousePos(x+300, y, 50, mousePos.x, mousePos.y)
  } else {
    // If the mouse is not here, look at 0, 0 for example...
    drawEyeThatFollowsMousePos(x+100, y, 50, 300, 300)
    drawEyeThatFollowsMousePos(x+300, y, 50, 300, 300)
  }
  
}	

 
    function getMousePos(canvas, evt) {
       
        var obj = canvas;
        var top = 0;
        var left = 0;
 
        while (obj && obj.tagName != 'BODY') {
            top += obj.offsetTop;
            left += obj.offsetLeft;
            obj = obj.offsetParent;
        }
       
        var mouseX = evt.clientX - left + window.pageXOffset;
        var mouseY = evt.clientY - top + window.pageYOffset;
        return {
            x:mouseX,
            y:mouseY
        };
     }
  
  
 function drawEyeThatFollowsMousePos(cx, cy, radius, mouseX, mouseY) {      
      // Draw external eye outline
      context.beginPath();
      context.arc(cx, cy, 50, 0, 2*Math.PI, false); 
      context.stroke();
      
      
      var angle = Math.atan2(cx-mouseX, cy-mouseY)
      
      
      var dist = distanceFromEyeCenterForDrawingEye(cx, cy, mouseX, mouseY, 15);
      
      
      var dx = dist*Math.sin(angle);
      var dy = dist*Math.cos(angle);
      
      
      context.beginPath();
      context.arc(cx-dx, cy-dy, 30, 0, 2*Math.PI, false);
      context.fill();
    }
    
    
    function distanceFromEyeCenterForDrawingEye(cx, cy, x, y, treshold) {
      var xd=(cx-x), yd=(cy-y);
      
      var distMouseEyeCenter = Math.sqrt(xd*xd + yd*yd);
      return Math.min(treshold, distMouseEyeCenter);
    }
  
</script>
</body onload="init();">
</html>

</body>
</html>
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