When looking at the parts list for the Arduino RGB LED spinning night light you must have noticed that current limiting resistors of different values were used for the Red and the Green/Blue pins of the RGB LED. That is due to them having different forward voltage ratings. You can find complete specs for the LED in the datasheet (when buying an electronic component you will have the option to download its datasheet, or the relevant information will be provided by the vendor).

We use Ohm’s Law to calculate current limiting resistor values:

Forward voltage ratings:

RED: 2.1V
GREEN: 3.3V
BLUE: 3.3V

Current:

I = 20mA

Supply voltage:

V = 5V

Ohm’s Law:

I = V/R => R = V/I

So for Red:

(5 – 2.1)/0.02 => R = 145 Ohm

For Green/Blue:

(5 – 3.3)/0.02 => R = 85 Ohm

As for the Arduino sketch, I chose to have the lamp fade between two colors, aqua (#00FFFF) and magenta (#FF00FF). For that I kept the Blue value at 255 and varied the Green and Red values between 0-255 to achieve the desired colors, as shown in the diagram:
(You can pick your favorite colors, cycle through the entire spectrum, or go psychedelic and show random colors with random delays)

// fade from aqua to magenta
  for (int i = 0; i < 256; i++) {
    analogWrite(RED, 255-i);
    analogWrite(GREEN, i);
    analogWrite(BLUE, 0);
    delay(50);
  }

  // fade from magenta to aqua
  for (int i = 0; i < 256; i++) {
    analogWrite(RED, i);
    analogWrite(GREEN, 255-i);
    analogWrite(BLUE, 0);
    delay(50);
  }

Here's the full sketch for the night light.

You might also enjoy:

1. Arduino RGB LED Spinning Night Light | Part 1
2. Arduino Motor Control for the Spinning Night Light | Part 3
3. Arduino RGB LED Spinning Night Light: Assembly | Part 2
4. Arduino LED Control Using DIP Switch | Part 1

1. Light: how do you feel today? Find out with the interactive, multi-function mood light: http://www.instructables.com/id/Interactive-Open-Source-Mood-Light/
2. Sound: have you ever played with water filled crystal glasses to make sound? How about using an Arduino Nano to control the music? Check out this cool video: http://www.vimeo.com/1261369
3. Game: it’s no Wii, but this simple version of Super Mario Bros using an 8×8 LED matrix rocks: http://vimeo.com/9928343
4. iPod: an Arduino-based iPod remote with a single button that toggles between Play and Pause on the iPod: http://davidfindlay.org/weblog/files/2009_09_07_ipod_remote.php

Buy the Arduino Nano Ver 3.0

The Nano at the official Arduino website: http://www.arduino.cc/en/Main/ArduinoBoardNano

You might also enjoy:

1. Arduino Mega: an Account of Coolness in 4 Links
2. Arduino Lilypad: an Account of Coolness in 4 Links
3. Arduino Mini: an Account of Coolness in 4 Links
4. An Arduino for Any Project