how to change buttons background from another activity ?

Question

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);
    }