I suggest to read the Technical FAQ, which tries to address precisely this kind of questions/errors.
In short, move the declaration of variables from draw() to the global level, replace the while by a simple if (ie. if attaining 400, exit(), for example) and let the successive calls to draw() to make the loop itself. This way, you will see the changes in the color of the square.

Hey phi.lho

thank you for your answer! I tried to understand and change my loop and ended up with a complete new but working script. I really tried hard but i was not able to complete the part where the color should be read because Processing always tells me that the method get(int,int) in the type Pimage is not applicable for the arguments (float, float). I really would appreciate your help with this ...

1. void setup()
   {
     size(400, 400);
     stroke(255); 
     Bilddatei = loadImage("my_localimage.jpg");
     image(Bilddatei, 0, 0);
   }
   float y = 10;
   float x = 10;
   
   void draw()
   {
     image(Bilddatei, 0, 0);
      
     if (x > 0 && x <380)
   { 
     rect(x,y, 25, 25);
     x = x + 50;    
   }
    
    else if (x > 370)
   {   
       y = y + 50;  
       x = 10;  
     rect(x,y, 25,25);  
   }
   color c = Bilddatei.get(x, y);
   // this is where the error occurs ...
       float redparts=red(c);
       float greenparts=green(c);
       float blueparts=blue(c);
       println("Red: "+ redparts);
       println("Green: "+ greenparts);
       println("Blue: "+ blueparts);
    
   } 
   

Thank you!
tm

1. int y = 0;
2. int x = 0;
3. PImage Bilddatei;
   
4. void setup() { 
5.   size(400, 400);  
6.   stroke(255);   
7.   Bilddatei = loadImage("my_localimage.jpg"); 
8.   image(Bilddatei, 0, 0);
9.   rect(x, y, 25, 25);
10. }
11. void draw() {  
12.   image(Bilddatei, 0, 0);     
13.   if (x < 400) {
14.     x = x + 50; 
15.     rect(x, y, 25, 25);
16.   } 
17.   else if (y < 400) {     
18.     y = y + 50;      
19.     x = 0; 
20.     rect(x, y, 25, 25);
21.   }
22.   color c = Bilddatei.get(x, y);// this is where the error occurs ...  
23.   float redparts=red(c);   
24.   float greenparts=green(c);   
25.   float blueparts=blue(c);   
26.   println("Red: "+ redparts);   
27.   println("Green: "+ greenparts);   
28.   println("Blue: "+ blueparts);
29. } 

Here is another example that might help.

1. int y = 0;
2. int x = 0;
3. PImage Bilddatei;
4. void setup() { 
5.   size(400, 400);  
6.   stroke(255);   
7.   Bilddatei = loadImage("my_localimage.jpg"); 
8.   image(Bilddatei, 0, 0);
9.   rect(x, y, 25, 25);
10. }
11. void draw() {  
12.   image(Bilddatei, 0, 0);   
13.   color c = Bilddatei.get(mouseX, mouseY);// this is where the error occurs ...    
14.   fill(c);
15.   rect(0, 0, 25, 25);
16.   float redparts=red(c);   
17.   float greenparts=green(c);   
18.   float blueparts=blue(c);   
19.   println("Red: "+ redparts);   
20.   println("Green: "+ greenparts);   
21.   println("Blue: "+ blueparts);
22. } 

Opps, I had something at the top there, but my code must have cleared it out.  It appeared to me that your X value and Y value were exceeding the size of your image (400x400).  Doing a get(420,500) for example would result in an error since those pixels don't exist.

Hello Jeff_g

thank you so much! I learned a lot from your example! I would not say that u understood everything in detail
but i am working on it ; ) What i do not understand: Everything works fine apart from displaying the correct color values.
I ran some tests and i was able to find out that the color value which is displayed is the one in the square on the
left of the actual square. (The image contains 8x8 different squares) Do you understand what i mean? Like some kind of offset.

Thank you once again,

tm

I guess part of the issue is that the box scanning the screen is a inaccurate portrayal of what is being read

1. rect(x, y, 25, 25);

Here you have a box that is 25 pixels wide, but your color reading only reads one pixel (x & y).  So that x & y need to be pretty exact with on the color that you're expecting.

1. color c = Bilddatei.get(x, y);

Maybe you want to target closer to the center of the rect.

1. color c = Bilddatei.get(x+12,y+12);

If you're reading one section over or one pixel over, you can adjust it in this way.