RBGLED.ino

// #define HC_05

#ifdef HC_05
  // Arduino + HC_05 altserial
  #define SERIAL_BAUD 19200
  #include "AltSoftSerial.h"
  AltSoftSerial altSerial;
  #define MySerial altSerial
#else
  // BLE NANO
  #define SERIAL_BAUD 19200
  #define MySerial Serial
#endif

// Common Anode I have

int red_light_pin= 3;
int green_light_pin = 5;
int blue_light_pin = 6;

void RGB_color(int red_light_value, int green_light_value, int blue_light_value)
 {
  analogWrite(red_light_pin, red_light_value);
  analogWrite(green_light_pin, green_light_value);
  analogWrite(blue_light_pin, blue_light_value);
}

void setup() {
  MySerial.begin(SERIAL_BAUD);

  pinMode(red_light_pin, OUTPUT);
  pinMode(green_light_pin, OUTPUT);
  pinMode(blue_light_pin, OUTPUT);
}

enum STATE {
  COMD,
  RVAL,
  GVAL,
  BVAL
};

uint8_t currentLED[] = { 0, 0, 0 };
uint8_t RGB_CMD[3];   // Stored by SCRATCH

// command format (4byte) : 0x81, R, G, B
boolean parseCmd(int c) {
  static int CURRENT_STATE = COMD;
  switch (CURRENT_STATE) {
    case COMD:
      if ( c == 0x81 )
        CURRENT_STATE = RVAL;
      return false;
    case RVAL:
      RGB_CMD[0] = c;
      CURRENT_STATE = GVAL;
      return false;
    case GVAL:
      RGB_CMD[1] = c;
      CURRENT_STATE = BVAL;
      return false;
    case BVAL:
      RGB_CMD[2] = c;
      CURRENT_STATE = COMD;
      return true;
  }
}

void serialEvent1() {
  while (MySerial.available()) {
    if ( parseCmd(MySerial.read()) ) {
      currentLED[0] = RGB_CMD[0];
      currentLED[1] = RGB_CMD[1];
      currentLED[2]  = RGB_CMD[2];
      RGB_color(255-currentLED[0], 255-currentLED[1], 255-currentLED[2]);
    }
  }
}

void loop() {
    delay(1000);
    MySerial.write(currentLED, 3);
    serialEvent1();
}

Yejin, if it is going with HC-05 with V4 firmware, no solution. It is fake HW issue.
If you mail me abut the trouble you face, I can give you an advice.

Yejin, if it is going with HC-05 with V4 firmware, no solution. It is fake HW issue. If you mail me abut the trouble you face, I can give you an advice.

I send my problem. This email : doojin.kang@gmail.com
Thank you :)