Turn RGB blinker via serial
Here is some untested code that uses a non-blocking timer to flash the leds.


uint8_t redLED   = 2;
uint8_t greenLED = 4;
uint8_t blueLED  = 3;

static uint8_t redRunning   = HIGH;   // leds off at start
static uint8_t greenRunning = HIGH;
static uint8_t blueRunning  = HIGH;

// static bool freq = 34;             // not sure what this is


void setup() {
  Serial.begin(115200);               // initialize the Serial
  Serial.write("started");

  pinMode(redLED,   OUTPUT);
  pinMode(greenLED, OUTPUT);
  pinMode(blueLED,  OUTPUT);

  digitalWrite(redLED,   HIGH);       // leds off
  digitalWrite(greenLED, HIGH);
  digitalWrite(blueLED,  HIGH);

//freq     = false;
  tick     = false;                   // time interval signal
  ledState = false;                   // state of led display

  static uint32_t ledTicker = millis();
}

void loop() {

  if (millis() - ledTicker &gt;= 125) {             // Once every 125 ms
    ledTicker = millis();
    tick = true;                                 // set the tick flag
    ledState = !ledState;                        // changes state at every tick 
  }

  if (Serial.available() &gt; 0) {
    String s = Serial.readStringUntil('\n');

//  String s = Serial.readStringUntil("#");      // Until CR (Carriage Return)
//  s.replace("#", "");
    Serial.println("s:" + s);


    if (s == "red_ON"   )  redRunning   = LOW ;  // sometimes, this format is easier to read
    if (s == "red_OFF"  )  redRunning   = HIGH;

    if (s == "green_ON" )  greenRunning = LOW ; 
    if (s == "green_OFF")  greenRunning = HIGH;

    if (s == "blue_ON"  )  blueRunning  = LOW ;
    if (s == "blue_OFF" )  blueRunning  = HIGH;
  }

  //
  if (tick) {                                    // 125 ms passed, so update the leds
    tick = false;                                // reset tick so that leds change only once every tick

    if (ledState) {                              // state of leds at this tick
      digitalWrite(redLED  , redRunning  );
      digitalWrite(greenLED, greenRunning);
      digitalWrite(blueLED , blueRunning );
    }  
    else {                                       // all off this time around
      digitalWrite(redLED  , HIGH);
      digitalWrite(greenLED, HIGH);
      digitalWrite(blueLED , HIGH);
    }
  }
}