Object Distance Sensing and Warning

int trigPin=3;
int echoPin=7;
int buzzPin=9;
int redPin=13;
int greenPin=2;
int bluePin=4;
int duration, cm;

void setup() {
  Serial.begin(9600);
  pinMode(trigPin, OUTPUT);
  pinMode(echoPin, INPUT);
  pinMode(buzzPin, OUTPUT); 
  pinMode(redPin, OUTPUT);
  pinMode(greenPin, OUTPUT);
  pinMode(bluePin, OUTPUT);
}

void loop() {
  digitalWrite(trigPin, LOW);
  delayMicroseconds(5);
  digitalWrite(trigPin, HIGH);
  delayMicroseconds(10);
  digitalWrite(trigPin, LOW);

  duration=pulseIn(echoPin, HIGH);
  cm=(duration/2)/29.1;
  Serial.print("distance in centimeters: ");
  Serial.println(cm);

  if(cm>30 || cm<0){
    digitalWrite(bluePin, LOW);
    digitalWrite(redPin, LOW);
    digitalWrite(greenPin, HIGH);
    delay(500);
    }  
  if(cm>=20 && cm<=30){
      digitalWrite(redPin, LOW);
      digitalWrite(greenPin, LOW);
      digitalWrite(bluePin, HIGH);
      delay(500);
    }
  if(cm<20 && cm>0){
    digitalWrite(greenPin, LOW);
    digitalWrite(bluePin, LOW);
    digitalWrite(redPin, HIGH);
    tone(buzzPin, 2000, 500);
    delay(1000);
    noTone(buzzPin);
    }
 
}