The following sketch contains various functions based on the CD77_Police_Lights_One functions, from my original sketch in GitHub, CD77_police_lights. It includes color change by button.

CD77_police_light_one_Button_Color_Change.ino

// Police_Lights_One with Button Color Control by Chemdoc77

// Includes new functions for various number of pixels that are on in a group and for forward and reverse directions along with the ability to change colors with a button.

// ******** Version 20190130 *************

/* This code is based on the police_lightsONE and police_lightsALL code:
in Funkboxing by Teldredge at:

  http://funkboxing.com/wordpress/wp-content/_postfiles/sk_qLEDFX_POST.ino

This sketch is one function from the following sketch in GitHub at:

  https://github.com/chemdoc77/CD77_FastLED/tree/master/CD77_police_lights

It uses the out of bounds concept and code from the FastLED XYMatrix Example at:

  https://github.com/FastLED/FastLED/blob/master/examples/XYMatrix/XYMatrix.ino

It uses Erin St Blaine's button code from: 

  https://learn.adafruit.com/dotstar-belly-dance-fans/overview

 */

#include "FastLED.h"

#define NUM_LEDS 24
#define Data_Pin 6


#define chipset NEOPIXEL 
#define BRIGHTNESS  40

int x = 0;  
int xx = NUM_LEDS-1;

//BUTTON SETUP STUFF

#define BUTTON_PIN 14 // this is an analog pin for a Teensy 3.1.  Use the appropiate analog pin for your MCU.
byte prevKeyState = HIGH;        
#define NUM_MODES 1  // note: modes start at 0.
int landingMode = 0;  //FIRST ACTIVE MODE



// From the FastLED XYMatrix Example out of bounds code:

CRGB leds_plus_safety_pixel[ NUM_LEDS + 4];
CRGB* const leds( leds_plus_safety_pixel + 4);



//=============================== 
 

void setup() { 
 delay(1000); // power-up safety delay 
 FastLED.addLeds<chipset, Data_Pin>(leds, NUM_LEDS); 
 FastLED.setBrightness(BRIGHTNESS);
 FastLED.setMaxPowerInVoltsAndMilliamps(5,1500); // sets max amperage to 1500 milliamps (1.5 amps)
 set_max_power_indicator_LED(13);
 
 // button setup stuff
 pinMode(BUTTON_PIN, INPUT_PULLUP);
  }

void loop(){  


switch (landingMode) {
       case 0: cd77_police_lights_one_modified_n_dots(2, 4, 60, CRGB::Red); break;            
       case 1: cd77_police_lights_one_modified_n_dots(2, 4, 60, CRGB::Blue); break;               
      
    }
    
// button management section
   
        byte currKeyState = digitalRead(BUTTON_PIN);
     
        if ((prevKeyState == LOW) && (currKeyState == HIGH)) {
            keyRelease();
        }
        
        prevKeyState = currKeyState;
    

}

//Sketch Functions  ================================

void cd77_police_lights_one_modified( int segments, int wait,CRGB color){
  EVERY_N_MILLIS(wait){ 
  x++;
  if (x >=  NUM_LEDS) {x = 0;}
  fill_solid(leds,NUM_LEDS, CRGB::Black); 
  for (int z=0; z< segments;z++){
  int y = x + (z*NUM_LEDS)/segments;
  if ((x + (z*NUM_LEDS)/segments) > NUM_LEDS-1) {y = (x + (z*NUM_LEDS)/segments)%NUM_LEDS; }
  leds[y]=   color;
 
}
  FastLED.show();    
   }   
}

//=======================

void cd77_police_lights_one_modified_2_dots( int segments, int wait,CRGB color){
  EVERY_N_MILLIS(wait){ 
  x++;
  if (x >=  NUM_LEDS) {x = 0;}
  fill_solid(leds,NUM_LEDS, CRGB::Black); 
  for (int z=0; z< segments;z++){
  int y = x + (z*NUM_LEDS)/segments;
  if ((x + (z*NUM_LEDS)/segments) > NUM_LEDS-1) {y = (x + (z*NUM_LEDS)/segments)%NUM_LEDS; }

  
  
  leds[y]=   color;
  leds[y-1]=   color;
 }
  FastLED.show();    
   }   
}

//======================

void cd77_police_lights_one_modified_revese( int segments, int wait,CRGB color){
  EVERY_N_MILLIS(wait){ 
  xx--;
  if (xx <=0) {xx = NUM_LEDS-1;}
  fill_solid(leds,NUM_LEDS, CRGB::Black); 
  for (int z=0; z< segments;z++){
  int y = xx + (z*NUM_LEDS)/segments;
  if ((xx + (z*NUM_LEDS)/segments) > NUM_LEDS-1) {y = (xx + (z*NUM_LEDS)/segments)%NUM_LEDS; }
  leds[y]=   color;
 
}
  FastLED.show();    
   }   
}


//==========================
void cd77_police_lights_one_modified_n_dots(int numb_dots, int segments, int wait,CRGB color){
  EVERY_N_MILLIS(wait){ 
  x++;
  if (x >=  NUM_LEDS) {x = 0;}
  fill_solid(leds,NUM_LEDS, CRGB::Black); 
  for (int z=0; z< segments;z++){
  int y = x + (z*NUM_LEDS)/segments;
  if ((x + (z*NUM_LEDS)/segments) > NUM_LEDS-1) {y = (x + (z*NUM_LEDS)/segments)%NUM_LEDS; }

  
  for (int dots=0; dots<numb_dots; dots++){
  //leds[y]=   color;
  leds[y-dots]=   color;
  }
 }
  FastLED.show();    
   }   
}

//====================

void cd77_police_lights_one_modified_n_dots_reverse(int numb_dots, int segments, int wait,CRGB color){
  EVERY_N_MILLIS(wait){ 
  xx--;
  if (xx <=0) {xx = NUM_LEDS-1;}
  fill_solid(leds,NUM_LEDS, CRGB::Black); 
  for (int z=0; z< segments;z++){
  int y = xx + (z*NUM_LEDS)/segments;
  if ((xx + (z*NUM_LEDS)/segments) > NUM_LEDS-1) {y = (xx + (z*NUM_LEDS)/segments)%NUM_LEDS; }

  
  for (int dots=0; dots<numb_dots; dots++){
  //leds[y]=   color;
  leds[y-dots]=   color;
  }
 }
  FastLED.show();    
   }   
}


//BUTTON CONTROL STUFF
// called when button is pressed
void shortKeyPress() {
    Serial.println("short");
    landingMode++;
    if (landingMode > NUM_MODES){
    landingMode=0; }
}

// called when key goes from pressed to not pressed
void keyRelease() {
    Serial.println("key release");
        shortKeyPress();
    }