const int BTN_PIN = 10;
const int GRN_LED_PIN = 12;
const int RED_LED_PIN = 11;

int oldBtnState = HIGH; // pullup
int ledState = HIGH;  // start with green on

// function returns true if button is pressed
bool checkButton()  {
  bool isPressed = false;

  int btnState = digitalRead(BTN_PIN);
  if (btnState != oldBtnState)  {
    oldBtnState = btnState;
    if (btnState == LOW)  {
      isPressed = true;
      Serial.println("Button pressed");
    }
    delay(20); // debounce
  }
  return isPressed;
}

void showLedState() {
  Serial.print("'ledState' is now: ");
  Serial.println(ledState ? "HIGH" : "LOW");
}

void setup() {
  Serial.begin(115200);
  pinMode(BTN_PIN, INPUT_PULLUP);
  pinMode(GRN_LED_PIN, OUTPUT);
  pinMode(RED_LED_PIN, OUTPUT);
  digitalWrite(GRN_LED_PIN, ledState);
  showLedState();
}

void loop() {
  bool isPushed = checkButton();
  if (isPushed) {
    ledState = !ledState;
    showLedState();
    digitalWrite(GRN_LED_PIN, ledState);
    digitalWrite(RED_LED_PIN, !ledState);
  }
}