Sparkfun Inventor's Kit Circuit-12

/*

  RGB_LED_Color_Fade_Cycle.pde
 
  Cycles through the colors of a RGB LED

  Written for SparkFun Arduino Inventor's Kit CIRC-RGB

*/
// LED leads connected to PWM pins
const int RED_LED_PIN = 9;
const int GREEN_LED_PIN = 10;
const int BLUE_LED_PIN = 11;

// Used to store the current intensity level of the individual LEDs
int redIntensity = 0;
int greenIntensity = 0;
int blueIntensity = 0;

// Length of time we spend showing each color
const int DISPLAY_TIME = 100; // In milliseconds


void setup() {
  // No setup required.
}
void loop() {
  // Cycle color from red through to green
  // (In this loop we move from 100% red, 0% green to 0% red, 100% green)
  for (greenIntensity = 0; greenIntensity <= 255; greenIntensity+=5) {
        redIntensity = 255-greenIntensity;
        analogWrite(GREEN_LED_PIN, greenIntensity);
        analogWrite(RED_LED_PIN, redIntensity);
        delay(DISPLAY_TIME);
  }

  // Cycle color from green through to blue
  // (In this loop we move from 100% green, 0% blue to 0% green, 100% blue) 
  for (blueIntensity = 0; blueIntensity <= 255; blueIntensity+=5) {
        greenIntensity = 255-blueIntensity;
        analogWrite(BLUE_LED_PIN, blueIntensity);
        analogWrite(GREEN_LED_PIN, greenIntensity);
        delay(DISPLAY_TIME);
  }

  // Cycle cycle from blue through to red
  // (In this loop we move from 100% blue, 0% red to 0% blue, 100% red)   
  for (redIntensity = 0; redIntensity <= 255; redIntensity+=5) {
        blueIntensity = 255-redIntensity;
        analogWrite(RED_LED_PIN, redIntensity);
        analogWrite(BLUE_LED_PIN, blueIntensity);
        delay(DISPLAY_TIME);
  }
}

http://ardx.org/HEXCOL

for hexadecimal colors
http://ardx.org/RGBMB

analogWrite(RED_LED_PIN, redIntensity); to analogWrite(RED_LED_PIN, redIntensity/3);