usbmidi library example with FastLED library.

teensy-usbmidi-code.ino

#include <FastLED.h>
#define NUM_LEDS 14
#define DATA_PIN 5      // Green
#define CLOCK_PIN 6     // Blue

CRGB leds[NUM_LEDS];

void OnProgramChange(byte channel, byte program) {
  delay(500);
  if (usbMIDI.getChannel() == 1 && usbMIDI.getData1() == 8) // MIDI CC 8, Channel 1
        {
          lightsOn();
        }
  else if (usbMIDI.getChannel() == 1 && usbMIDI.getData1() == 9) // MIDI CC 8, Channel 1
        {
          
          lightsOff();
        }         
}

void setup()
{
  FastLED.addLeds<APA102, DATA_PIN, CLOCK_PIN>(leds, NUM_LEDS);
  FastLED.show();
  usbMIDI.setHandleProgramChange(OnProgramChange);
}

void loop()
{
  usbMIDI.read();
}


// function fading lights on
void lightsOn() {
  int lightsOnMaxBrightness = 64;
  int lightsOnMinBrightness = 2;

  for (int i = lightsOnMinBrightness; i < lightsOnMaxBrightness; i++) {
    lightsOnMinBrightness = i;
    FastLED.show();
    for ( int j = 0; j < NUM_LEDS; j++) {
      leds[j] = CRGB(lightsOnMinBrightness);
    }
  }
}

void lightsOff() {
  int lightsOffMaxBrightness = 64;
  int lightsOffMinBrightness = 2;

  for (int i = lightsOffMaxBrightness; i > lightsOffMinBrightness; i--) {
    lightsOffMaxBrightness = i;
    FastLED.show();
    for ( int j = 0; j < NUM_LEDS; j++) {
      leds[j] = CRGB(lightsOffMaxBrightness);
    }
  }
}