#include <FastLED.h>
#include "FastLED_RGBW.h"

#define NUM_LEDS 21
CRGBW leds[NUM_LEDS];
CRGB *ledsRGB = (CRGB *) leds;

void setup() {
  FastLED.addLeds<WS2812B, 3, RGB>(ledsRGB, getRGBWsize(NUM_LEDS));
  FastLED.addLeds<WS2812B, 4, RGB>(ledsRGB, NUM_LEDS);
}

void loop() {
  fill_palette(leds, NUM_LEDS, millis()/8, 2, RainbowColors_p, 255, LINEARBLEND);
  FastLED[0].showLeds();

  // shuffle GRBW -> RGB and show on 2nd strip
  for (auto i = 0; i < NUM_LEDS; i++)
    ledsRGB[i] = CRGB(leds[i].r, leds[i].g, leds[i].b);
  FastLED[1].showLeds();
}