because you are overlaying a green red and blue semi transparent ellipse.resulting in grey.
see the different if they are slightly offset.
what exactly do you want to do ?

void draw () {
  background(255);
    noFill ();
    fill ( 200, 0, 0, 50 );
    ellipse ( mouseX, mouseY+100, 100, 100 );
   
    noFill ();
    fill ( 0, 200, 0, 50 );
    ellipse ( mouseX, mouseY-100, 100, 100 );
   
    noFill ();
    fill ( 0, 0, 200, 50 );
    ellipse ( mouseX, mouseY, 100, 100 );
    }

Explained, thanks. I'm just testing, as I'm new to graphics programming (used to study only data programming). My initial intention was to draw circles of the three colors, one after another. Can you see some way of doing it?

its never a good idea to use delay.
you should do it differently.
could you explain what "one after another" means in your case. in time? next to each other?

To display things sequentially (I should really finish this tutorial!), you have to display one thing, wait some frames, display the next thing, etc.
By "wait some frames", I mean not to use delay(), which is indeed a bad idea (display is updated only at the end of draw()), but to do nothing or to display the same thing for a given number of frames or seconds.
Get the frame count with the frameCount variable. Get the time with the millis() function call.

Thanks, your answers complement each other. 

Ok, now I got it doing what was my intention:

the draw doesnt get unsynced with the frameCount, thats actually the same.

but you can write it much shorter.

take a look at modulo : http://processing.org/reference/modulo.html

void draw(){
    if(frameCount%3==0)      fill ( 255, 0, 0, 80 );
    if(frameCount%3==1)      fill ( 0, 255, 0, 80 );
    if(frameCount%3==2)      fill ( 0, 0, 255, 80 );
    ellipse ( mouseX, mouseY, 100, 100 );
}

1. int dframe1 = 7;
   int dframe2 = dframe1*2;
   int dframe3 = dframe1*3;
2. void setup () {
     size (800, 800);
     smooth ();
     background (255);
     stroke (0, 60);
     rectMode(CENTER);
   }
3. void draw () { 
     if ((frameCount % dframe3)==0) {
       fill ( 255, 0, 0, 60 );
       ellipse ( mouseX, mouseY, 100, 100 );
     }
     if ((frameCount % dframe3)==dframe1) {
       fill ( 0, 255, 0, 60 );
       triangle(mouseX-66, mouseY+33+17, mouseX+66, mouseY+33+17, mouseX, mouseY-66+17);
     }
     if ((frameCount % dframe3)==dframe2) {
       fill ( 0, 0, 255, 60 );
       rect ( mouseX, mouseY, 100, 100 );
     }
   }

@cedrickiefer Thanks again for the answer and tip on writing it shorter.

i thought he would like to make use of the mouse wheel not the mousewheel button.
not sure though.
but here is an example on how to use the mouse wheel.


// mouseWheel.pde
// code to use mouse wheel in Processing

// must import java.awt.event.*
import java.awt.event.*;

float y;

void setup() {
  size(400,400);

  // add mouse wheel listener
  frame.addMouseWheelListener(new MouseWheelInput());

  y=height/2;
}

void draw() {
  background(100);
  ellipse(width/2, y, 50,50);
}

// convenience class to listen for MouseWheelEvent
// modify mouseWheelMoved() for your own purposes

class MouseWheelInput implements MouseWheelListener{

  void mouseWheelMoved(MouseWheelEvent e) {
    int step=e.getWheelRotation();
    y=y+step*5;
  }

}

@alphacap I was really asking about the rotation of the wheel, but you antecipated another doubt of mine, because I didn't know how to get which mouse button was pressed. Thanks

@cedrickiefer Java code seems greek for me. I know some about C/C++, will java be difficult to learn? (it seems...)
Can I use the code you posted in my program, as I really don't know other way to implement it? An is there some way to know if the wheel was turned u or down? Thanks

Thank you all who helped, here is my final first processing program: