void setup () {
  //Output pin initialization for the LEDs
  pinMode (Led_Red, OUTPUT); 
  pinMode (Led_Green, OUTPUT); 
  pinMode (Led_Blue, OUTPUT); 
}
void loop () {
   // In this for-loop, the 3 LEDs will get different PWM-values
   // Via mixing the brightness of the different LEDs, you will get different colors. 
   for (val = 255; val> 0; val--)
      {
       analogWrite (Led_Blue, val);
       analogWrite (Led_Green, 255-val);
       analogWrite (Led_Red, 128-val);
       delay (1);
   }
   // You will go backwards through the color range in this second for loop.
   for (val = 0; val <255; val++)
      {
      analogWrite (Led_Blue, val);
      analogWrite (Led_Green, 255-val);
      analogWrite (Led_Red, 128-val);
      delay (1);
   }
}