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.

Photos

Illuminated diffuser

Unplugged diffuser