Need help with timer project

I'm just learning and I'm having trouble with the code and getting this error message (exit status 1 lvalue required as left operand of assignment). I'm trying to create a system where when the start_button is pushed, it turns on the GreenLED then an amount of time goes by and turns off the GreenLED and turns on the YellowLED. Then the RedLED. Kind of like a stop light but I can't use delay because I have two more buttons I need to monitor. I'm not going to add the other buttons until I get this part of the code working. Any help would be greatly appreciated.

const int ledPin =  3;      // the number of the LED pin

// Variables will change:
int ledState = LOW;             // ledState used to set the LED
long previousMillis = 0;        // will store last time LED was updated
boolean start_state = false;    // Start Button status set to false until button is pressed
// the following variables is a long because the time, measured in miliseconds,
// will quickly become a bigger number than can be stored in an int.
long interval_1 = 300;           // interval between green and yellow led}
long interval_2 = 300;           // inteval between yellow and red led)

#define GreenLED 2
#define YellowLED 3
#define RedLED 4
#define start_button 8
#define emergency_button 9
//#define open_button 10   ***This switch is unused****

void setup() {

  // set the digital pin as output:
  pinMode(GreenLED, OUTPUT); 
  pinMode(YellowLED, OUTPUT);
  pinMode(RedLED, OUTPUT); 
  pinMode(start_button, INPUT);//Input for Start Button
  pinMode(emergency_button, INPUT);//Input for Emergency Button 


void loop()
  // check to see if it's time to blink the LED; that is, if the 
  // difference between the current time and last time you blinked 
  // the LED is bigger than the interval at which you want to 
  // blink the LED.
 digitalWrite(RedLED = LOW)
  unsigned long currentMillis = millis();
  if (digitalRead(start_button) == HIGH)//If Start Button is pressed
    start_state = true;
    if (start_state == HIGH);
    digitalWrite(GreenLED = HIGH);

  if(currentMillis - previousMillis > interval_1) {
    // save the last time you blinked the LED 
    previousMillis = currentMillis;   
    digitalWrite(GreenLED = LOW);
    digitalWrite(YellowLED = HIGH);

   if(currentMillis - previousMillis > interval_2) {
    // save the last time you blinked the LED 
    previousMillis = currentMillis; 
    digitalWrite(YellowLED = LOW);
    digitalWrite(RedLED = HIGH);

devilstang:    digitalWrite(GreenLED = HIGH);

Should be

digitalWrite(GreenLED, HIGH);

And all your other digitalWrite() calls need the same change. Please read

There are multiple other problems with your code but I'll let you see if you can fix those. Here's a tip: Always use Tools > Auto Format, the automatic indentation makes it easier to spot bugs.

You should assign the millis() as “unsigned long” and not just “long”, to avoid the variable value rolling over when millis() gives a value greater than the range of “long” data type

Thanks for all of the responses. I'll try your suggestions and report back.