char mychar;

void setup()
{
    Serial.begin(9600);
    pinMode(7,OUTPUT);
    pinMode(8,OUTPUT);
    pinMode(9,OUTPUT);
}

void loop()
{
    Serial.println("Enter which colour you want to Blink\n1.'g' for green colour \n2.'y' for yellow colour\n3.'r' for red colour");

    while(Serial.available() >= 0)
    {
        mychar = Serial.read();
        if (mychar == 'r')
        {
            digitalWrite(7, HIGH);
            delay(100);
            digitalWrite(7, LOW);
            delay(100);
            digitalWrite(8, LOW);
            digitalWrite(9, LOW);
         }
         if (mychar == 'y')
         { 
             digitalWrite(7, LOW);
             digitalWrite(8, HIGH);
             delay(100);
             digitalWrite(8, LOW);
             delay(100);

             digitalWrite(9, LOW);
             break;
          }  
          if (mychar == 'g')
          {
              digitalWrite(7, LOW);
              digitalWrite(8, LOW);
              digitalWrite(9, HIGH);
              delay(100);
              digitalWrite(9, LOW);
              delay(100);
          }
    }
}