RGB color mixer on an Arduino Uno

// RGB color mixer: one RGB LED, three PWM pins, every color in between.
//
//   pin 9   the red leg, through 220 ohm
//   pin 10  the green leg, through 220 ohm
//   pin 11  the blue leg, through 220 ohm
//   the long leg (the common cathode) to GND
//
// An RGB LED is three LEDs under one lens. analogWrite() sets how bright each
// one is, from 0 (off) to 255 (full), and your eye mixes the three into one
// color. Red and green make yellow, green and blue make cyan, all three make
// white.
//
// The sketch fades from one named color to the next over one second, holds it
// for a second, and prints its name and its three numbers when it gets there.
//
// # Why there are two whites
//
// The red die needs about 1.8 V and the green and blue ones about 3 V. On the
// same 220 ohm resistors from a 5 V pin, red gets about 15 mA and the other
// two about 10, so "all three at 255" comes out pinkish. The balanced white
// turns red down to two thirds to make up for it. On a real LED you can do
// the same in code, or give red a bigger resistor.
//
// # Common anode
//
// This is a common-cathode LED: HIGH lights a color. On a common-anode LED
// the long leg goes to 5 V and LOW lights a color, so every value is flipped:
// write 255 - value instead.

const int RED_PIN = 9;
const int GREEN_PIN = 10;
const int BLUE_PIN = 11;

struct Color {
  const char *name;
  uint8_t r, g, b;
};

const Color COLORS[] = {
    {"red", 255, 0, 0},
    {"orange", 255, 40, 0},
    {"yellow", 255, 160, 0},
    {"green", 0, 255, 0},
    {"cyan", 0, 255, 255},
    {"blue", 0, 0, 255},
    {"purple", 120, 0, 255},
    {"magenta", 255, 0, 255},
    {"white (all 255)", 255, 255, 255},
    {"balanced white", 170, 255, 255},
};
const int COUNT = sizeof(COLORS) / sizeof(COLORS[0]);

const int FADE_STEPS = 50;
const int STEP_MS = 20;
const int HOLD_MS = 1000;

int r = 0, g = 0, b = 0;

void show(int red, int green, int blue) {
  analogWrite(RED_PIN, red);
  analogWrite(GREEN_PIN, green);
  analogWrite(BLUE_PIN, blue);
}

void fadeTo(const Color &c) {
  for (int step = 1; step <= FADE_STEPS; step++) {
    show(r + (c.r - r) * step / FADE_STEPS,
         g + (c.g - g) * step / FADE_STEPS,
         b + (c.b - b) * step / FADE_STEPS);
    delay(STEP_MS);
  }
  r = c.r;
  g = c.g;
  b = c.b;
}

void setup() {
  pinMode(RED_PIN, OUTPUT);
  pinMode(GREEN_PIN, OUTPUT);
  pinMode(BLUE_PIN, OUTPUT);
  Serial.begin(115200);
  Serial.println("Mokxi Uno: RGB color mixer");
}

void loop() {
  for (int i = 0; i < COUNT; i++) {
    fadeTo(COLORS[i]);
    Serial.print(COLORS[i].name);
    Serial.print(": ");
    Serial.print(COLORS[i].r);
    Serial.print(", ");
    Serial.print(COLORS[i].g);
    Serial.print(", ");
    Serial.println(COLORS[i].b);
    delay(HOLD_MS);
  }
}