WS2812B Fastled Theater chase

Other Hardware LEDs and Multiplexing
16

That's right : the line FastLED.show(); was not correctly placed too (I can't run the code here, so I' m kind of blind when checking it) :

#include <FastLED.h>

// Define the array of leds
#define LED_DT 6
#define COLOR_ORDER GRB
#define LED_TYPE WS2812
#define NUM_LEDS 150
uint8_t max_bright = 255;
struct CRGB leds[NUM_LEDS];


void setLEDs (int K, int N) {
  FastLED.clear();
  for (int i = 0; i < NUM_LEDS; i++) {
    if (i % N == K) leds[i].setRGB (127, 0, 0); // set red LED
  }
  FastLED.show();
}

void setup() {
  delay (1000);
  LEDS.addLeds<LED_TYPE, LED_DT, COLOR_ORDER>(leds, NUM_LEDS);
  FastLED.setBrightness(max_bright);
}

void loop() {
  int N = 5;
  for  (int i = 0; i < N; i++) {
    setLEDs (i, N);
    delay (50);
  }
}

You can play with colors in the function, for example :

void setLEDs (int K, int N) {
  FastLED.clear();
  for (int i = 0; i < NUM_LEDS; i++) {
    int red = 255 * (i % N) / (N - 1);
    int blu = 255 - red;
    if (i % N == K) leds[i].setRGB (red, 0, blu); // set changing color
  }
  FastLED.show();
}

(unless I did other mistakes) this should chnage the color of the lit LEDs from red to blue, then back to red again.