fade byserial.read

hello, i need to introduce fade in this programme bytyping it in on serial monitor. I already tried some arrays and other things can someone help me?
With this program I type the first colour and the right colour work.

//r=rood
//g= groen
//b=blauw

int rood = 5;
int groen = 6;
int blauw = 7;

void setup() {

Serial.begin(9600);
pinMode(rood, OUTPUT);
pinMode(groen, OUTPUT);
pinMode(blauw, OUTPUT);
}
char rx_byte = 0;

void loop() {
if (Serial.available () > 0) { ///is a char available
rx_byte = Serial.read();
switch (rx_byte) {
case 114:
digitalWrite(rood, HIGH);
digitalWrite(groen, LOW);
digitalWrite(blauw, LOW);
Serial.println("rood");
break;

case 103:
digitalWrite(rood, LOW);
digitalWrite(groen, HIGH);
digitalWrite(blauw, LOW);
Serial.println("groen");
break;

case 98:
digitalWrite(rood, LOW);
digitalWrite(groen, LOW);
digitalWrite(blauw, HIGH);
Serial.println("blauw");
break;
default:
Serial.println("Invalid option");
break;
}
}

}

Please read How to use this forum - please read. - Installation & Troubleshooting - Arduino Forum and pay special attention to point #7. Next apply it to your post.

To get you ideas for serial communication, you can read the updated Serial Input Basics thread.