// Black - ground wire
// Red - Power supply wire
// short leg on LED is cathode and should be connected to the ground

//Define LEDs
const int red = 12;
const int yellow = 11;
const int green = 10;
const int Button = 2;
// define time delays
int time_delay = 300;

int LEDchecker = 1;

void setup() {
  // put your setup code here, to run once:
  pinMode(red, OUTPUT);
  pinMode(yellow, OUTPUT);
  pinMode(green, OUTPUT);
  pinMode(Button, INPUT);

  //must initialise pins all as off initally
  initialise_pins();
}

void loop() {
  // put your main code here, to run repeatedly:
  if (digitalRead(Button) == LOW){
   disco_lights();
  }
}

/* if you wanted to be able to pause as soon as Button = HIGH, you could check the Button reading
between each delay
if (digitalRead(Button) == LOW){
  digitalWrite(yellow, LOW);
  digitalWrite(red, HIGH);
  digitalWrite(green, HIGH);
  delay(time_delay);
}
if (digitalRead(Button) == LOW){
  digitalWrite(red, LOW);
  digitalWrite(green, LOW);
  digitalWrite(yellow, HIGH);
  delay(time_delay);
}
*/

// another method

void disco_lights(){
  if (digitalRead(Button) == LOW){
    if (LEDchecker == 1){
      digitalWrite(yellow, LOW);
      digitalWrite(red, HIGH);
      digitalWrite(green, HIGH);
      LEDchecker = 2;
    }
    else{
      digitalWrite(red, LOW);
      digitalWrite(green, LOW);
      digitalWrite(yellow, HIGH);
      LEDchecker = 1;  
    }
    delay(time_delay);
  }
}
//*/


// function to initialise pins
void initialise_pins(){
  digitalWrite(green, LOW);
  digitalWrite(yellow, LOW);
  digitalWrite(red, LOW);
}
/*
// function for traffic lights
void disco_lights(){
  digitalWrite(red, HIGH);
  digitalWrite(green, HIGH);
  delay(time_delay);
  
  digitalWrite(red, LOW);
  digitalWrite(green, LOW);
  digitalWrite(yellow, HIGH);
  delay(time_delay);
  
  digitalWrite(yellow, LOW);  
}
*/

// your method and the method used are both correct, but the method used pauses the 
// blinking mid blink, whilst yours waited for the loop to end.