blue gradient

#include <FastLED.h>

#define NUM_LEDS  18
#define LED_PIN   2

CRGB leds[NUM_LEDS];

uint8_t paletteIndex = 0;

CRGBPalette16 purplePalette = CRGBPalette16 (
    CRGB::DarkViolet,
    CRGB::DarkViolet,
    CRGB::DarkViolet,
    CRGB::DarkViolet,

    CRGB::Magenta,
    CRGB::Magenta,
    CRGB::Linen,
    CRGB::Linen,

    CRGB::Magenta,
    CRGB::Magenta,
    CRGB::DarkViolet,
    CRGB::DarkViolet,

    CRGB::DarkViolet,
    CRGB::DarkViolet,
    CRGB::Linen,
    CRGB::Linen
);

void setup() {
  FastLED.addLeds<WS2812B, LED_PIN, GRB>(leds, NUM_LEDS);
  FastLED.setBrightness(50);
}

void loop() {
  fill_palette(leds, NUM_LEDS, paletteIndex, 255 / NUM_LEDS, purplePalette, 255, LINEARBLEND);

  EVERY_N_MILLISECONDS(10){
    paletteIndex++;
  }

  FastLED.show();
}