Unintended, instant LED Power-UP right after turning on 12V SMPS

[jim-p]

Remove the relay and connect the LED cathodes to ground.
Connect a 4.7K resistor between pins 9, 10, 11 and ground, one resistor on each pin.

Add this code in setup

digitalWrite (LED_R, LOW);
digitalWrite (LED_G, LOW);
digitalWrite (LED_B, LOW);

Place it right after the pinMode statements for those pins

[mikedb]

@gr_monk

FastLED.addLeds<WS2812B, STRIP_DIN>(led, STRIP_LED_NUM);
  for (int i = 0; i < STRIP_LED_NUM; i++) {
    led[i] = CRGB::White;
    FastLED.show();
    delay(50);

You have this in setup so Leds will turn on ?