Serial flush - seems I can't get it to clear data

Projects Programming
3

I am trying to get rid of a delay if there's no serial data being sent for example:

void colours(){
  fill_solid(leds, NUM_LEDS, CRGB(varR1, varG1, varB1)); //put this here so when we start or break out of fx, colour is populated 
  
  if (Serial.available() > 0) {
    dataIn = Serial.readString();
    delay(100);//give the data time to be read before we can read info from it.
    
    if (dataIn.startsWith("A")) {
      String stringR = dataIn.substring(dataIn.indexOf("R") + 1, dataIn.indexOf("G"));
      varR1 = stringR.toInt();
      String stringG = dataIn.substring(dataIn.indexOf("G") + 1, dataIn.indexOf("B"));
      varG1 = stringG.toInt();
      String stringB = dataIn.substring(dataIn.indexOf("B") + 1, dataIn.indexOf("E"));
      varB1 = stringB.toInt();
      if ((stringR.length() > 0) && (stringG.length() > 0)&& (stringB.length() > 0)){
        fill_solid(leds, NUM_LEDS, CRGB(varR1, varG1, varB1));
        }
      /*
      //debug - to check incoming remote commands work.
      char buf [100]; // must be large enough for the whole string
      sprintf (buf, "Red: %d, Green: %d, Blue: %d", varR1, varG1, varB1);
      String sum = (buf);
      Serial.println(sum);
      */
      }
    
    else if (dataIn.startsWith("B")) {
      String stringBright = dataIn.substring(dataIn.indexOf("E") + 1, dataIn.indexOf("F"));
      BRIGHTNESS = stringBright.toInt();
      FastLED.setBrightness(BRIGHTNESS);

      /*
      //debug - to check incoming remote commands work.
      char buf [100]; // must be large enough for the whole string
      sprintf (buf, "Brightness: %d", BRIGHTNESS);
      String sum = (buf);
      Serial.println(sum);
      */
      
      }
    else if (dataIn.startsWith("FX")) {
      FastLED.clear();
      ledMode = 1;
      }
  }
  
  if (dataIn.length() > 0){
    Serial.print(dataIn);
    dataIn = ""; 
  }
  delay(250); //this seems to be the best time to allow fastled code to run and give enough time for data to be sent to the leds.
}

If I change the code and put the delay here:

 if (dataIn.length() > 0){
    Serial.print(dataIn);
    delay(250); //this seems to be the best time to allow fastled code to run and give enough time for data to be sent to the leds.
    dataIn = ""; 
  }

The delay doesn't seem to work properly and only the first 2 string characters are read. I realised after posting it wasn't the serial buffer I needed to flush, I just needed to clear the dataIn string. Now I just have this delay issue.