internal pull up help

hi,
I got this code going for 4 colour andon light system. It's pretty basic when Red switch is on Red bulb is turned on and buzzer sounded, and for other other switches, it's the same.

I have used the internal pull-up resistor for input pins. When I connect them through a switch to the ground the input pin turns low. everything works fine when I connect it to a 6V switchmode power supply at home. but when I connect the same supply to my workplace the inputs get triggered at random order by themselves.

I soldered four 1k resistors to the inputs and other ends to 5V. them it worked fine. But when I CONNECTED 10K resistors inputs triggered by themselves like earlier.

can someone explain what is going on?
does the input pull-up resistor have limitations?
Is it a good idea to connect external pull-up resistors anyway, if so what value?

/*

   four color andon light system

   four input switches for four output bulbes and buzzers
   when input R is ON and other inputs are OFF output bulb R is blinked and buzzer R is sounded
   When Two or more inputs are ON all bulbes and buzzes according to thoes switches are activated.

*/

// declarations
const int REDSwitchPin = 2;
const int GREENSwitchPin = 3;
const int BLUESwitchPin = 4;
const int YELLOWSwitchPin = 5;


const int REDBulbPin = 6;
const int GREENBulbPin = 7;
const int BLUEBulbPin = 8;
const int YELLOWBulbPin = 9;


const int buzz = 10;
//const int GREENBuzzerPin = 11;
//const int BLUEBuzzerPin = 12;
//const int YELLOWBuzzerPin = 13;

// variables to hold input pin satus
int REDSwitchInput = 0;
int GREENSwitchInput = 0;
int BLUESwitchInput = 0;
int YELLOWSwitchInput = 0;

void setup() {
  // put your setup code here, to run once:
  Serial.begin(9600);

  pinMode(REDSwitchPin, INPUT_PULLUP);
  pinMode(GREENSwitchPin, INPUT_PULLUP);
  pinMode(BLUESwitchPin, INPUT_PULLUP);
  pinMode(YELLOWSwitchPin, INPUT_PULLUP);

  pinMode(REDBulbPin, OUTPUT);
  pinMode(GREENBulbPin, OUTPUT);
  pinMode(BLUEBulbPin, OUTPUT);
  pinMode(YELLOWBulbPin, OUTPUT);

  pinMode(buzz, OUTPUT);
  //  pinMode(GREENBuzzerPin, OUTPUT);
  //  pinMode(BLUEBuzzerPin, OUTPUT);
  //  pinMode(YELLOWBuzzerPin, OUTPUT);

  digitalWrite(buzz, LOW);
  //  digitalWrite(GREENBuzzerPin, LOW);
  //  digitalWrite(BLUEBuzzerPin, LOW);
  //  digitalWrite(YELLOWBuzzerPin, LOW);

  digitalWrite(REDBulbPin, HIGH);
  digitalWrite(GREENBulbPin, HIGH);
  digitalWrite(BLUEBulbPin, HIGH);
  digitalWrite(YELLOWBulbPin, HIGH);

}

void loop() {
  // read inputs
  REDSwitchInput = digitalRead(REDSwitchPin);
  GREENSwitchInput = digitalRead(GREENSwitchPin);
  BLUESwitchInput = digitalRead(BLUESwitchPin);
  YELLOWSwitchInput = digitalRead(YELLOWSwitchPin);

  //  Serial.print(REDSwitchInput);
  //  Serial.print("     ");
  //  Serial.print(GREENSwitchInput);
  //  Serial.print("     ");
  //  Serial.print(BLUESwitchInput);
  //  Serial.print("     ");
  //  Serial.println(YELLOWSwitchInput);



  if (REDSwitchInput == LOW && GREENSwitchInput == LOW && BLUESwitchInput == LOW && YELLOWSwitchInput == LOW) {
    RED_GREEN_BLUE_YELLOW();
  }

  if (REDSwitchInput == LOW && GREENSwitchInput == LOW && BLUESwitchInput == LOW && YELLOWSwitchInput == HIGH) {
    RED_GREEN_BLUE();
  }

  if (REDSwitchInput == LOW && GREENSwitchInput == LOW && BLUESwitchInput == HIGH && YELLOWSwitchInput == LOW) {
    RED_GREEN_YELLOW();
  }

  if (REDSwitchInput == LOW && GREENSwitchInput == LOW && BLUESwitchInput == HIGH && YELLOWSwitchInput == HIGH) {
    RED_GREEN();
  }

  if (REDSwitchInput == LOW && GREENSwitchInput == HIGH && BLUESwitchInput == LOW && YELLOWSwitchInput == LOW) {
    RED_BLUE_YELLOW();
  }

  if (REDSwitchInput == LOW && GREENSwitchInput == HIGH && BLUESwitchInput == LOW && YELLOWSwitchInput == HIGH) {
    RED_BLUE();
  }

  if (REDSwitchInput == LOW && GREENSwitchInput == HIGH && BLUESwitchInput == HIGH && YELLOWSwitchInput == LOW) {
    RED_YELLOW();
  }

  if (REDSwitchInput == LOW && GREENSwitchInput == HIGH && BLUESwitchInput == HIGH && YELLOWSwitchInput == HIGH) {
    RED();
  }

  if (REDSwitchInput == HIGH && GREENSwitchInput == LOW && BLUESwitchInput == LOW && YELLOWSwitchInput == LOW) {
    GREEN_BLUE_YELLOW();
  }

  if (REDSwitchInput == HIGH && GREENSwitchInput == LOW && BLUESwitchInput == LOW && YELLOWSwitchInput == HIGH) {
    GREEN_BLUE();
  }

  if (REDSwitchInput == HIGH && GREENSwitchInput == LOW && BLUESwitchInput == HIGH && YELLOWSwitchInput == LOW) {
    GREEN_YELLOW();
  }

  if (REDSwitchInput == HIGH && GREENSwitchInput == LOW && BLUESwitchInput == HIGH && YELLOWSwitchInput == HIGH) {
    GREEN();
  }

  if (REDSwitchInput == HIGH && GREENSwitchInput == HIGH && BLUESwitchInput == LOW && YELLOWSwitchInput == LOW) {
    BLUE_YELLOW();
  }

  if (REDSwitchInput == HIGH && GREENSwitchInput == HIGH && BLUESwitchInput == LOW && YELLOWSwitchInput == HIGH) {
    BLUE();
  }

  if (REDSwitchInput == HIGH && GREENSwitchInput == HIGH && BLUESwitchInput == HIGH && YELLOWSwitchInput == LOW) {
    YELLOW();
  }

  if (REDSwitchInput == HIGH && GREENSwitchInput == HIGH && BLUESwitchInput == HIGH && YELLOWSwitchInput == HIGH) {
    ALLOFF();
  }

}

To start with, 6V isn't a good choice of supply voltage. It is too high to be connected to directly to the microcontroller via the 5V pin, but it is lower then you should use on the VIN pin (or barrel jack). Try with a more appropriate supply and see how it behaves then.

Sounds very much like EMI.

Without circuit diagram and photos of the actual contraption not much more I can say about it.

The basic problem is that stray signals are coupling capacitively to your wiring. This injects small current
spikes into the signal, and these current spikes become voltage spikes across the pullup resistor. The
larger the resistor value, the larger the voltage spikes for a given amount of interference.

So lowering the resistor value reduces the problem.

Another approach is to add a capacitor to ground on the signal wire. Then the current spikes also have to
charge up the capacitor. If each current spike has a total charge of Q, the voltage step that generates across
a capacitor C is Q/C. Thus a larger capacitor will reduce the voltage effects of a given spike.

Try 10 to 100nF in the first instance.