Arduino sketch to cycle an RGB LED through the colour spectrum.

rgb_spectrum.c

const int redPin = 8;
const int greenPin = 9;
const int bluePin = 10;
#define LDR 0

void setup() {
  // Start off with the LED off.
  setColourRgb(0,0,0);
  Serial.begin(9600);
    unsigned int rgbColour[3];

  // Start off with red.
  rgbColour[0] = 255;
  rgbColour[1] = 0;
  rgbColour[2] = 0;  

  // Choose the colours to increment and decrement.
  for (int decColour = 0; decColour < 3; decColour += 1) {
    int incColour = decColour == 2 ? 0 : decColour + 1;

    // cross-fade the two colours.
    for(int i = 0; i < 255; i += 1) {
      rgbColour[decColour] -= 1;
      rgbColour[incColour] += 1;
      
      setColourRgb(rgbColour[0], rgbColour[1], rgbColour[2]);
      delay(5);
    }
  }
}

void loop() {

}

void setColourRgb(unsigned int red, unsigned int green, unsigned int blue) {
  analogWrite(redPin, red);
  analogWrite(greenPin, green);
  analogWrite(bluePin, blue);
  Serial.print("rgb(");
  Serial.print( red );
    Serial.print(",");
  Serial.print( green );
    Serial.print(",");
  Serial.print( blue );
  Serial.print(") -");
  Serial.print( analogRead( LDR ) );
  Serial.print("-");
    Serial.println();
 }