how to change buttons background from another activity ?

hi, i have 3 XML layout file (in res) with several buttons . i use xml layouts in view pager into main activity in my app ... now i want change buttons background when my condition is true , how do it ?

this my button in XML layout :

<ir.rahgoshafan.alphabetpuzzle.customs.PuzzleButton
android:id="@+id/btn_green455"
android:layout_width="0dp"
android:layout_height="wrap_content"
android:layout_weight="0.33"
android:onClick="BTN_CLICK"
android:background="@drawable/button_red"
android:contentDescription="@string/app_name"
android:text="?????? ?????"
android:textColor="#fff"
android:textSize="20sp"
android:textStyle="bold" />

and view pager in main activity :

private void init() {
ViewPager VIEW_PAGER = (ViewPager) findViewById(R.id.tour_view_pager);
DOTS_LAYOUT = (LinearLayout) findViewById(R.id.tour_layout_dots);
LAYOUTS = new int[]{R.layout.items_one, R.layout.items_two, R.layout.items_three};
addBottomDots(0);
TourViewPagerAdapter ViewPagerAdapter = new TourViewPagerAdapter();
VIEW_PAGER.setAdapter(ViewPagerAdapter);
VIEW_PAGER.addOnPageChangeListener(viewPagerPageChangeListener);
}