This program uses arrow keys to change colors on a lamp.
The colors are within the color spectrum

I have used the following code to map HSV values to RGB
The r and b change values from 0 to 360 and vice versa to show 360 different colors

*/

// The three primary colour LEDs, driven as analgue outputs (actually PWM, but
// close enough for our analogue eyes).

int ledPinRed = 9; // Red LED connected to analogue out pin
int ledPinGrn = 10; // Green LED connected to analogue out pin
int ledPinBlu = 11; // Blue LED connected to analogue out pin
// Constants to define the ranges.

const int hueRedLow = 0;
const int hueRedHigh = 255;
const int hueBlue = 170;

// The size of the angle of one sector (1/6 of a colour wheel), and of a complete
// cycle of the colour wheel.

const int angleMin = 0;
const int angleSector = 60;
const int angleMax = 360;
//range of brighness from 0 to 255

const int brightMin = 0;
const int brightMax = 255;

// The hue is the range 0 (red) to 255 (blue)
// The brightness ranges from 0 (dark) to 255 (full brightness)

int hue, brightness;

// The saturation is fixed at 255 (full) to remove blead-through of different
// colours.
// It could be linked separately if a demonstration of hue is desired.

const int saturation = 255;

// The brightess of each LED (0 to 255).
//defining the start of the color wheel
int color = 0;

unsigned int r, g, b;

void setup() {
// Setting a baud rate for communication via USB.
Serial.begin(9600);
Serial.println("Welcome to the Spectrumizer");
Serial.println("Use the 'r' key to go towards red in the color wheel");
Serial.println("Use the 'b' key to go towards blue in the color wheel");

// Set LED pins to output.
pinMode(ledPinRed, OUTPUT);
pinMode(ledPinGrn, OUTPUT);
pinMode(ledPinBlu, OUTPUT);
}

void loop() {

int key = 0;
// The brightness is fixed at full for the colour wheel.
brightness = 255;
//using serial.read function to read keystrokes from the keyboard
if (Serial.available() > 0) {
// read the incoming byte:
key = Serial.read();
}

if(key == 114)
{
if(color != 0)
{
for( int i = 0 ;i <10;i++)
{
color = color - 1;
// Colour wheel (red to red, wrapped around in a cycle).
hue = map(color, angleMin, angleMax, hueRedLow, hueRedHigh);
// Do the conversion.
HSBToRGB(hue, saturation, brightness, &r, &g, &b);

analogWrite(ledPinRed, r);
analogWrite(ledPinGrn, g);
analogWrite(ledPinBlu, b);
delay(50);
}
}
}
else if(key == 98)
{
if(color != 360)
{
for( int i = 0 ;i <10;i++)
{
color = color + 1;
// Colour wheel (red to red, wrapped around in a cycle).
hue = map(color, angleMin, angleMax, hueRedLow, hueRedHigh);
// Do the conversion.
HSBToRGB(hue, saturation, brightness, &r, &g, &b);

analogWrite(ledPinRed, r);
analogWrite(ledPinGrn, g);
analogWrite(ledPinBlu, b);
delay(50);
}
}
}
}

// This function taken from here:
// http://eduardofv.com/read_post/179-Arduino-RGB-LED-HSV-Color-Wheel-

void HSBToRGB(
unsigned int inHue, unsigned int inSaturation, unsigned int inBrightness,
unsigned int *oR, unsigned int *oG, unsigned int *oB )
{
if (inSaturation == 0)
{
// achromatic (grey)
*oR = *oG = *oB = inBrightness;
}
else
{
unsigned int scaledHue = (inHue * 6);
unsigned int sector = scaledHue >> 8; // sector 0 to 5 around the color wheel
unsigned int offsetInSector = scaledHue - (sector << 8); // position within the sector
unsigned int p = (inBrightness * ( 255 - inSaturation )) >> 8;
unsigned int q = (inBrightness * ( 255 - ((inSaturation * offsetInSector) >> 8) )) >> 8;
unsigned int t = (inBrightness * ( 255 - ((inSaturation * ( 255 - offsetInSector )) >> 8) )) >> 8;