Color-changing RGB LED by rotating hue

hsv_rgb_led.ino

#include <Keyboard.h>
#include <math.h>

#define BLUE 3
#define GREEN 5
#define RED 6

void setup()
{
  pinMode(RED, OUTPUT);
  pinMode(GREEN, OUTPUT);
  pinMode(BLUE, OUTPUT);
  digitalWrite(RED, HIGH);
  digitalWrite(GREEN, HIGH);
  digitalWrite(BLUE, LOW);
}

int redValue;
int greenValue;
int blueValue;

// See https://stackoverflow.com/a/8510751.
double m[3][3] = {
  {0.9998984634375941, -0.010025383273369482, 0.010126919835775301},
  {0.010126919835775301, 0.9998984634375941, -0.010025383273369482},
  {-0.010025383273369482, 0.010126919835775301, 0.9998984634375941}};

int clamp(double v) {
  if (v < 0) return 0;
  if (v > 255) return 255;
  return int(v + 0.5);
}

void apply(int* r, int* g, int* b) {
  double ro = *r, go = *g, bo = *b;
  *r = clamp(ro * m[0][0] + go * m[0][1] + bo * m[0][2]);
  *g = clamp(ro * m[1][0] + go * m[1][1] + bo * m[1][2]);
  *b = clamp(ro * m[2][0] + go * m[2][1] + bo * m[2][2]);
}

void loop() {
  redValue = 255;
  greenValue = 255;
  blueValue = 0;

  for(int i = 0; i < 360; i += 1) {
    analogWrite(RED, redValue);
    analogWrite(GREEN, greenValue);
    analogWrite(BLUE, blueValue);
    delay(10);
    apply(&redValue, &greenValue, &blueValue);
  }
}