Azend's colour fader modified for a smooth progression using the power of trigonometry.

gistfile1.ino

/* Diyode CodeShield Pinout Constants */
#define ENCODER_A 14
#define ENCODER_B 15
#define ENCODER_PORT PINC
#define SWITCH 13
#define BUTTON 12
#define RGB_RED 11
#define RGB_GREEN 10
#define RGB_BLUE 9
#define LED 6
#define SERVO 5
#define PIEZO 3
#define RELAY 2
#define POT 2
#define HALL 3
#define THERMISTOR 4
#define PHOTOCELL 5
 
double angle = 0;
 
void setup () {
pinMode( RGB_RED, OUTPUT );
pinMode( RGB_GREEN, OUTPUT );
pinMode( RGB_BLUE, OUTPUT );
}
 
void loop () {
  analogWrite( RGB_RED, byte( sin(angle) * 127 + 128));
  analogWrite( RGB_GREEN, byte( sin(angle + PI * 2/3) * 127 + 128 ) );
  analogWrite( RGB_BLUE, byte( sin(angle - PI * 2/3) * 127 + 128 ) );

  delay (15);
  angle+= 0.01;
  if( angle > PI * 2 ) angle = 0;
}