#define outArrSize 100
String incomingData;
char outData[outArrSize];

void setup() {
  Serial.begin(115200);
  Serial.println("ready for Data:\n");
}

void loop() {

  if (Serial.available() > 0) {
    incomingData = Serial.readStringUntil(':');
    incomingData.toCharArray(outData, outArrSize);
    Serial.write(outData);
  }
}