#include <FastLED.h>
    #define NUM_LEDS 50
    #define DATA_PIN 2
    CRGB leds[NUM_LEDS];
     void setup() {
       FastLED.addLeds<WS2811, DATA_PIN>(leds, NUM_LEDS);
FastLED.setBrightness( 25 );
   }
   void loop() {
    for(int n=0; n<NUM_LEDS; n+=2)
        leds[n] = CRGB::Blue;
   for(int n=1; n<=NUM_LEDS; n+=2)
        leds[n] = CRGB::Green;
        FastLED.show();
    }