this should do the trick:

var rect = new Rectangle(10, 10, 10, 10);
var rectPath = document.createRectangle(rect);
rectPath.style.fill.color = RGBColor(0,1,0);

thank you jonathan, but your code doesn't work for me...

I have to put you code into this function:

function drawAlien(x1, y1, size, colorato){
var nero = new CMYKColor(0, 0, 0, 1);

var rect = new Rectangle(x1, x2, size, size);
rect.center = new Point(x2, y2);
activeDocument.createRectangle(rect);
}

thank you, now I'll try your code and I'll post feedback!

Yes! it works thank you very very much!

I'll try to go further with my script idea (this is only a part of it)...
I hope I'll can post something soon!