Holi color mixer on an Arduino Uno

// Holi color-mixing light: three knobs mix red, green and blue light into any
// color on a ring of LEDs, and a button throws a splash of bright colors
// across it, the way gulal powder flies at Holi.
//
//   knobs     three potentiometers, wipers -> A0 (red), A1 (green), A2 (blue)
//   button    pin 2 -> button -> GND        (INPUT_PULLUP)
//   ring      12 WS2812B LEDs, DIN <- 330 ohms <- pin 6
//
// Holi, the festival of colors, welcomes spring with friends and family
// throwing colored powder and water at each other until everyone is every
// color at once. Light mixes differently from powder: paint and powder get
// darker as you mix them, but light adds up. Red and green light make yellow,
// red and blue make magenta, green and blue make cyan, and all three make
// white. Turn the knobs and see.
//
// The splash picks random colors from a palette of bright Holi shades and
// lands each one on a random LED, then the ring fades back to your mix.
//
// Things to try. Turn red and green up with blue down: yellow. Press the
// button for a splash. Add your favorite color to SPLASH.

#include <Adafruit_NeoPixel.h>

const int BUTTON_PIN = 2;
const int LEDS = 12;
Adafruit_NeoPixel ring(LEDS, 6, NEO_GRB + NEO_KHZ800);

const uint32_t SPLASH[] = {0xFF1493, 0xFFD700, 0x00C853, 0x2979FF, 0xFF6D00, 0xAA00FF, 0x00E5FF};
const int SHADES = sizeof(SPLASH) / sizeof(SPLASH[0]);

uint8_t splashLeft[LEDS];   // frames each LED keeps its splash color
uint32_t splashColor[LEDS];
uint32_t lastMix = 0xFFFFFFFF;
int splashes = 0;

void setup() {
  Serial.begin(9600);
  pinMode(BUTTON_PIN, INPUT_PULLUP);
  ring.begin();
  ring.setBrightness(160);
  Serial.println("Holi color mixer: turn the knobs, press for a splash");
}

void loop() {
  uint8_t r = analogRead(A0) / 4;
  uint8_t g = analogRead(A1) / 4;
  uint8_t b = analogRead(A2) / 4;
  uint32_t mix = ring.Color(r, g, b);

  if (digitalRead(BUTTON_PIN) == LOW) {
    splashes++;
    for (int n = 0; n < 8; n++) {
      int i = random(0, LEDS);
      splashColor[i] = SPLASH[random(0, SHADES)];
      splashLeft[i] = random(20, 60);
    }
    Serial.print("Splash ");
    Serial.println(splashes);
    while (digitalRead(BUTTON_PIN) == LOW) delay(10);
  }

  for (int i = 0; i < LEDS; i++) {
    if (splashLeft[i] > 0) {
      splashLeft[i]--;
      ring.setPixelColor(i, splashColor[i]);
    } else {
      ring.setPixelColor(i, mix);
    }
  }
  ring.show();

  // Report the mix when it changes by more than a little.
  if ((mix >> 20) != (lastMix >> 20) || ((mix >> 12) & 0xF) != ((lastMix >> 12) & 0xF) || ((mix >> 4) & 0xF) != ((lastMix >> 4) & 0xF)) {
    lastMix = mix;
    Serial.print("Mix red ");
    Serial.print(r);
    Serial.print(" green ");
    Serial.print(g);
    Serial.print(" blue ");
    Serial.println(b);
  }
  delay(20);
}