When called without any parameters, get() will return an image of the sketch's window.
But if you specify some parameters, get(x,y), it will return a color instead.
The parameters you specify define a point, (x,y), on the sketch window.

The color returned will be the color at that point.

So the real question is, which pixel are you trying to get the color of?

1. color grab = get( ???, ??? ); // Replace ???'s with the x and y positions of the point that you want to get the color of.

There are better ways of only updating the sketch at specific times. Instead of using framerate(), only redraw your scene after a certain number of milliseconds has elapsed (which you can check with the millis() function.

The call to loop() is not needed. The draw() function is called over and over again already by default. The call to loop() does nothing but slow your program down - in a bad way.

Finally, what exactly are you trying to accomplish? If you could tell us what your program's expected behavior is, we would be able to help you a lot more. You full code currently accomplishes the same as this code:

ok, the purpose of this program is to generate a random image, colour an area previously defined.

In this section of the code I want to grab all the pixel color in the screen and assign them to a single red greeb blu variable.

 

"generate a random image"
What kind of a random image?

Each pixel is a random color?

The background is a random, solid color?

A random picture is pulled from the web and displayed?

"colour an area"

A rectangular area? What color?

"previously defined"

Defined where? How? By who?

"I want to grab all the pixel color in the screen"
That's an image. It's a 2D array of many pixels' colors.

"and assign them to a single red greeb blu variable."
How do you plan to convert a 2D array of colors to a single color?

Are you going to average them?

Sum the amounts of each primary color?

What do you plan to use the result for?

Q: "generate a random image"

A: Each pixel is a random color

Q: "colour an area"

A: The entire area of the window

Q: "previously defined"

A: by the size of the window

Q:"and assign them to a single red greeb blu variable."

A: I want to obtain the value of the color splitted in the red green blu value 

    example: Orange (#FF8103) have R=255 G=129 B=3

Q:What do you plan to use the result for?

A: I want to elaborate the result of "Orange" (in the example) and return a different color in another random position

1. PImage saved_back;
2. 
   
3. void setup() {
4.   // Define the sketch's size.
5.   size(220, 220);
6.   // Draw a randomly generated background where each pixel is a random color.  
7.   for (int x=0;x<width;x++) {
8.     for (int y=0;y<height;y++) {
9.       stroke(random(255), random(255), random(255) );
10.       point(x, y);
11.     }
12.   }
13.   // Remember that background as an image.
14.   saved_back = get(); // The get() returns an image.
15.   // Make it look pretty.
16.   smooth();
17.   ellipseMode(CENTER);
18. }
19. 
    
20. void draw() {
21.   // Draw the saved background.
22.   image( saved_back, 0, 0 ); 
23.   // Get the color where the mouse is.
24.   color c = get(mouseX, mouseY); // This get is given a position in the image, so it returns a color.
25.   // Draw a circle at the mouse's location.
26.   stroke(0); // The circle has a black border.
27.   fill(c); // The circle is drawn in the specified color.
28.   ellipse(mouseX, mouseY, 20,20);
29. }
30. 
    
31. void mousePressed(){
32.   // When the mouse is pressed, give details about the current color.
33.   color c = get(mouseX, mouseY);
34.   println( "At (" + mouseX + ", " + mouseY + "), the color is R: " + int(red(c)) + " G: " + int(green(c)) + " B: " + int(blue(c)) );
35. }

ok, I've modified the source to and in the example linked below I want to draw a white square, I grab the rgb value of all the pixel in the window area, and cause the square have all rgb value at max the checksfondo() routine fill the area by black square, until this black square are more than the 70% of the window area.

sorry for bad english....:-)

http://pastebin.com/U9pXUZzL