Trying to add a previous button to the project but it isn't working. Just asking to be pointed in the right direction

public class FunFactsActivity extends AppCompatActivity implements OnClickListener {
    //Declare our view variables
    private TextView factTextView;
    public static final String TAG = FunFactsActivity.class.getSimpleName();
    private ColorWheel colorWheel = new ColorWheel();
    private Button showFactButton;
    private Button previousFactButton;
    private FactBook factBook = new FactBook();
    private RelativeLayout relativeLayout;
    public static final List<Integer> factNumbers = new ArrayList<Integer>();
    public static final List<Integer> previousColors = new ArrayList<Integer>();

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_fun_facts);

        //Assign the Views from the layout file to corresponding variables
        factTextView = findViewById(R.id.factTextView);
        relativeLayout = (RelativeLayout) findViewById(R.id.relativeLayout);
        showFactButton = findViewById(R.id.showFactButton);
        showFactButton.setOnClickListener(this);
        previousFactButton = findViewById(R.id.previousFactButton);
        previousFactButton.setOnClickListener(this);
    }

    @Override
    public void onClick(View view) {
        switch (view.getId()) {
            case R.id.showFactButton:
                String fact = factBook.getFact();
                //update the screen with our new fact
                factTextView.setText(fact);

                int color = colorWheel.getColor();
                relativeLayout.setBackgroundColor(color);
                showFactButton.setTextColor(color);
                break;
            case R.id.previousFactButton:
                fact = factBook.getPreviousFact();
                factTextView.setText(fact);

                color = colorWheel.getPreviousColor();
                relativeLayout.setBackgroundColor(color);
                previousFactButton.setTextColor(color);
                break;
            default:
                break;
        }
    }
}

class FactBook {
    //Fields or Member Variables - Properties about the object
    private final String[] facts = {
            "Ants stretch when they wake up in the morning.",
            "Ostriches can run faster than horses.",
            "Olympic gold medals are actually made mostly of silver.",
            "You are born with 300 bones; by the time you are an adult you will have 206.",
            "It takes 8 minutes and 20 seconds for light from the Sun to reach Earth.",
            "Some bamboo plants can grow almost a meter in just one day.",
            "The state of Florida is bigger than England.",
            "Some penguins can leap 2-3 meters out of the water.",
            "On average, it takes 66 days to form a new habit.",
            "Mammoths still walked the earth when the Great Pyramid was being built." };


    // Methods - Actions the object can take


    String getFact() {

        //Randomly select a fact
            Random randomGenerator = new Random();
            int randomNumber = randomGenerator.nextInt(facts.length);
            factNumbers.add(randomNumber);
            return facts[randomNumber];

        }
    String getPreviousFact(){
        //select the previous fact
     int n = factNumbers.size();
        n -= 1;
        int x = factNumbers.get(n);
        return facts[x];
    }
}

public class ColorWheel {

    //Fields or Member Variables - Properties about the object
    private final String[] colors = {
            "#39add1", // light blue
            "#3079ab", // dark blue
            "#c25975", // mauve
            "#e15258", // red
            "#f9845b", // orange
            "#838cc7", // lavender
            "#7d669e", // purple
            "#53bbb4", // aqua
            "#51b46d", // green
            "#e0ab18", // mustard
            "#637a91", // dark gray
            "#f092b0", // pink
            "#b7c0c7", // light gray
            "#03c4eb"  // DanzerPress
    };


    // Methods - Actions the object can take

   int getColor() {

        //Randomly select a fact
        Random randomGenerator = new Random();
        int randomNumber = randomGenerator.nextInt(colors.length);
        int color = Color.parseColor(colors[randomNumber]);
        previousColors.add(color);
        return color;
    }

    int getPreviousColor(){
       //get previous fact color scheme
        int n = previousColors.size();
        n -= 1;
        int prevColor = previousColors.get(n);
        return prevColor;
    }

}