Code

/*
 * Serial RGB LED
 * ---------------
 * Serial commands control the brightness of R,G,B LEDs
 *
 * Command structure is "<colorCode><colorVal>", where "colorCode" is
 * one of "r","g",or "b" and "colorVal" is a number 0 to 255.
 * E.g. "r0"   turns the red LED off. 
 *      "g127" turns the green LED to half brightness
 *      "b64"  turns the blue LED to 1/4 brightness
 *      A string of 'r' increments the red LED by 5 per 'r'
 *      Ditto 'g' and 'b'
 *
 * Created 18 October 2006
 * copyleft 2006 Tod E. Kurt <tod@todbot.com
 * http://todbot.com/
 *
 * Modified 12 September 2007 by Andrew McDiarmid
 */

char serInString[10];   // array that will hold the different bytes of the string. 10=10characters;
                        // -> you must state how long the array will be else it won't work properly
char colorCode;
int colorVal;

int redPin   = 9;   // Red LED,   connected to digital pin 9
int greenPin = 10;  // Green LED, connected to digital pin 10
int bluePin  = 11;  // Blue LED,  connected to digital pin 11

int redValue;        //These are used to track the current levels of the LEDs
int greenValue;
int blueValue;

void setup() {
  pinMode(redPin,   OUTPUT);   // sets the pins as output
  pinMode(greenPin, OUTPUT);  
  pinMode(bluePin,  OUTPUT);
  Serial.begin(9600);
  analogWrite(redPin,   127);   // set them all to mid brightness
  redValue = 127;
  analogWrite(greenPin, 127);   // set them all to mid brightness
  greenValue = 127;
  analogWrite(bluePin,  127);   // set them all to mid brightness
  blueValue = 127;
  Serial.println("Enter color command (e.g. 'r043' or 'rrrr', max 10 characters):"); 
}

void loop () {
  //read the serial port and create a string out of what you read
  readSerialString(serInString);
   
  if (serInString[1] >= 48 && serInString[1] <= 57) {                //runs this code if second character is an integer character
    colorCode = serInString[0];
    if( colorCode == 'r' || colorCode == 'g' || colorCode == 'b' ) {
      colorVal = atoi(serInString + 1);
      Serial.print("setting color ");
      Serial.print(colorCode);
      Serial.print(" to ");
      Serial.print(colorVal);
      Serial.println();
     
      if(colorCode == 'r') {
        analogWrite(redPin, colorVal);
        redValue = colorVal;
      }
      else if(colorCode == 'g') {
        analogWrite(greenPin, colorVal);
        greenValue = colorVal;
      }
      else if(colorCode == 'b'){
        analogWrite(bluePin, colorVal);
        blueValue = colorVal;
      }
     for(int i = 0 ; i <= 9; i+=1){        // indicates we've used this string and zeroes all characters in the string
       serInString[i] = 0;
     }                                          
    }
    Serial.print(serInString);
  }
   
    else if(serInString[0] != 0){                                //runs this code if the second character is not an integer character (see above)
      int j = 0;                                                 //and if the first character is not the null character
     
      while (j < 10 & serInString[j] != 0) {                     //iterates through the string, incrementing LEDs as indicated by the characters found
        if(serInString[j] == 'r') {
          redValue = (redValue + 5) % 256;                       //Incrementing the appropriate LED's value
          analogWrite(redPin, redValue);
        }
        if(serInString[j] == 'g') {
          greenValue = (greenValue + 5) % 256;
          analogWrite(greenPin, greenValue);
        }
        if(serInString[j] == 'b') {
          blueValue = (blueValue + 5) % 256;
          analogWrite(bluePin, blueValue);
        }
        j++;
      }
      Serial.print("Red is now ");
      Serial.print(redValue);
      Serial.print("; green is now ");
      Serial.print(greenValue);
      Serial.print("; blue is now ");
      Serial.print(blueValue);
      Serial.println(".");
     
      for(int i = 0 ; i <= 9; i+=1){        // indicates we've used this string and zeroes all characters in the string
       serInString[i] = 0;
     }
    }
  
  delay(100);  // wait a bit, for serial data
}

//read a string from the serial and store it in an array
//you must supply the array variable

void readSerialString (char *strArray) {
  int i = 0;
  if(!Serial.available()) {
    return;
  }
  while (Serial.available()) {
    strArray[i] = Serial.read();
    i++;
  }
}

Diffuser

I chose to reflect downwards onto a sheet of white paper.  The colors mix well this way, with the exception of red, which just isn't that bright.  The reflector is the top to a lava lamp.

Photo