I used Arduino with three LEDs (red, blue and green). I edited the sketch code to turn on the LED with keyboard interaction.  Each enter of the corresponding key is 255/10 of the brightness. I also use bubble rap to create an air balloon as a diffuser. I then turn the red and blue LED to 127 and green LED to zero to make a purple air balloon. However, the result is not as expected. Blue and Red are still somehow separated. Lastly, I  uploaded the code and took pictures of the result.

Components

• 1 Arduino
• 3 LED
• 3 Resistor (220Ω)
• 1 Breadboard
• One difuser

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
 *
 * Created 18 October 2006
 * copyleft 2006 Tod E. Kurt <tod@todbot.com
 * http://todbot.com/
 */

char serInString[100]; // array that will hold the different bytes of the string. 100=100characters;
 // -> 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

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
 analogWrite(greenPin, 127); // set them all to mid brightness
 analogWrite(bluePin, 127); // set them all to mid brightness
 Serial.println("Please enter color corresponding key to change the brightness (i.e. 'r' for RED; ''g' for GREEN; and 'b' for BLUE :"); 
}

void loop () {
 // clear the string
 memset(serInString, 0, 100);
 //read the serial port and create a string out of what you read
 readSerialString(serInString);
 
 colorCode = serInString[0];
 if( colorCode == 'r' || colorCode == 'g' || colorCode == 'b' ) {
 colorVal = atoi(serInString+1);
 colorVal = strlen(serInString);
 colorVal = (colorVal*255/10); // increase the value by 10% by calculating the number of times you type r, g or b defines the brightness (% of)
 if (colorVal>255)
 { // need a way to turn off colors
 colorVal = 0;}
 Serial.print("setting color ");
 Serial.print(colorCode);
 Serial.print(" to ");
 Serial.print(colorVal);
 Serial.println();
 serInString[0] = 0; // indicates we've used this string
 if(colorCode == 'r') 
 analogWrite(redPin, colorVal);
 else if(colorCode == 'g')
 analogWrite(greenPin, colorVal);
 else if(colorCode == 'b')
 analogWrite(bluePin, colorVal);
 }
 
 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++;
 }
}