Change the text color of the menu when selecting an Android fragment

public class MainActivity extends AppCompatActivity implements View.OnClickListener {

    Button btn[] = new Button[4];
    ViewPager viewPager = null;
    Thread thread = null;
    Handler handler = null;
    intp=0; //page number
    int v=1; //Screen Switching Bangee

    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        //viewPager
        viewPager = (ViewPager)findViewById(R.id.viewPager);
        MyViewPagerAdapter adapter = new MyViewPagerAdapter(getSupportFragmentManager());

        viewPager.setAdapter(adapter);

        btn[0] = (Button)findViewById(R.id.btn_a);
        btn[1] = (Button)findViewById(R.id.btn_b);
        btn[2] = (Button)findViewById(R.id.btn_c);
        btn[3] = (Button)findViewById(R.id.btn_d);

        for(int i=0;i<btn.length; i++){
            btn[i].setOnClickListener(this);
        }
    }


    @Override
    public void onClick(View v) {

        switch(v.getId()){
            case R.id.btn_a:
                viewPager.setCurrentItem(0);
                Toast.makeText (this "A Button", Toast.LENGTH_SHORT).show();
                break;
            case R.id.btn_b:
                viewPager.setCurrentItem(1);
                Toast.makeText (this "B button", Toast.LENGTH_SHORT).show();
                break;
            case R.id.btn_c:
                viewPager.setCurrentItem(2);
                Toast.makeText (this "C Button", Toast.LENGTH_SHORT).show();
                break;
            case R.id.btn_d:
                viewPager.setCurrentItem(3);
                break;
            default:
                break;

        }

    }
}