Button controlling Servo

servo.ino

#include <Servo.h>

Servo servo;

int btnPin = 2;
int lightPins[] = {3, 5, 6};
int serPin = 11;
int pos = 0;

void setup()
{
  servo.attach(serPin);
  
  pinMode(btnPin, INPUT);
  
  for(int a = 0; a<3; a++) {
    pinMode(lightPins[a], OUTPUT);
  }
  
}

void loop()
{
  int a = digitalRead(btnPin);
  if(a == 1) {
    green();
    pos++;
    pos = min(180, pos);
  } 
  else {
    red();
    pos--;
    pos = max(0, pos);
  }

  servo.write(pos);
  delay(15);
}

void red()
{
  analogWrite(lightPins[0], 255);
  analogWrite(lightPins[1], 0);
  analogWrite(lightPins[2], 0);
}

void green()
{
  analogWrite(lightPins[0], 0);
  analogWrite(lightPins[1], 255);
  analogWrite(lightPins[2], 0);
}