I am in the process of creating something like MS Paint where the use can click on a color and draw small ellipses using that color. It only works for the last color (blue)..

1. boolean drawing = false;
   
   // setup - runs one time
   void setup()
     {
     // set a stage size of 500 x 500 pixels
     size (500,500);
   
     // smooth all drawing
     smooth();
   
     // don't draw an outline around your shapes
     noStroke();
     }
   
   // draw - runs once a frame
   void draw()
     {
     // draw a color button
    
     fill (255,0,0);
     rect(0,0,100,50);
    
     fill (0,255,0);
     rect(100,0,100,50);
   
     fill (0,0,255);
     rect(200,0,100,50);
    
     // are we in draw mode?
     if (drawing == true)
       {
       ellipse(mouseX, mouseY, 30, 30);
       }
   
     }
   
   // mousePressed - runs every time the mouse is pressed
   void mousePressed()
     {
     // did we click the button?
     if (mouseX > 0 && mouseX < 100 && mouseY > 0 && mouseY < 50)
       {
       fill (255,0,0);
   
       }
   
     if (mouseX > 100 && mouseX < 200 && mouseY > 0 && mouseY < 50)
       {
       fill (0,255,0);
   
   
       }
   
     if (mouseX > 200 && mouseX < 300 && mouseY > 0 && mouseY < 50)
       {
       fill (0,0,255);
   
       }
         
   
     // otherwise we are in draw mode
     else
       {
       drawing = true;
       }
     }
   
   
   
   
   void mouseReleased()
     {
     drawing = false;
     }