How to change the background color of a view from a different activity in android?

I am working on a Quiz app. First when a user opens the app they go to the MainActivity, from there when they press start they go to the Categories Activity , from there after selecting a category they go to the Sets Activity, from there after selecting a set the go to the Questions Activity and finally after completing all the questions they reach the Score Activity. Here in the score activity when the click on Done button they are redirected to the MainActivity. In the Score Activity i want to change the color of the Set that they completed to green instead of the default color. How can i do this? I created a sets item layout xml file and used an adapter to fill the gridview in the Sets Activity with views from the adapter. Currently i am getting a null object reference after clicking the Done button in the ScoreActivity.

Here is the code :

SetsAdapter.java

public class SetsAdapter extends BaseAdapter {

    private int numOfSets;

    public SetsAdapter(int numOfSets) {
        this.numOfSets = numOfSets;
    }

    @Override
    public int getCount() {
        return numOfSets;
    }

    @Override
    public Object getItem(int position) {
        return null;
    }

    @Override
    public long getItemId(int position) {
        return 0;
    }

    @Override
    public View getView(final int position, View convertView, final ViewGroup parent) {

        View view;
        if(convertView == null){
            view = LayoutInflater.from(parent.getContext()).inflate(R.layout.set_item_layout, parent, false);
        }
        else {
            view = convertView;
        }

        view.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent questionIntent = new Intent(parent.getContext(), QuestionActivity.class);
                questionIntent.putExtra("SETNUM", position +1);
                parent.getContext().startActivity(questionIntent);
            }
        });

        ((TextView) view.findViewById(R.id.setNumber)).setText(String.valueOf(position+1));

        return view;
    }
}

SetsActivity.java

public class SetsActivity extends AppCompatActivity {

    private GridView sets_grid;
    private FirebaseFirestore firestore;
    public static int categoryID;
    private Dialog loadingDialog;

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

        Toolbar toolbar = (Toolbar)findViewById(R.id.set_toolbar);
        setSupportActionBar(toolbar);
        String title = getIntent().getStringExtra("CATEGORY");
        categoryID = getIntent().getIntExtra("CATEGORY_ID",1);
        getSupportActionBar().setTitle(title);
        getSupportActionBar().setDisplayHomeAsUpEnabled(true);

        sets_grid = findViewById(R.id.sets_gridView);

        loadingDialog = new Dialog(SetsActivity.this);
        loadingDialog.setContentView(R.layout.loading_progressbar);
        loadingDialog.setCancelable(false);
        loadingDialog.getWindow().setBackgroundDrawableResource(R.drawable.progress_background);
        loadingDialog.getWindow().setLayout(ViewGroup.LayoutParams.WRAP_CONTENT, ViewGroup.LayoutParams.WRAP_CONTENT);
        loadingDialog.show();

        firestore = FirebaseFirestore.getInstance();
        loadSets();



    }

    private void loadSets() {
        firestore.collection("Quiz").document("CAT" + String.valueOf(categoryID))
                .get().addOnCompleteListener(new OnCompleteListener<DocumentSnapshot>() {
            @Override
            public void onComplete(@NonNull Task<DocumentSnapshot> task) {
                if (task.isSuccessful()) {
                    DocumentSnapshot doc = task.getResult();

                    if (doc.exists()) {
                        long sets = (long) doc.get("SETS");
                        SetsAdapter adapter = new SetsAdapter(Integer.valueOf((int)sets));


                        sets_grid.setAdapter(adapter);


                    } else {
                        Toast.makeText(SetsActivity.this, "No Sets Exists!", Toast.LENGTH_SHORT).show();
                        finish();

                    }
                } else {
                    Toast.makeText(SetsActivity.this, task.getException().getMessage(), Toast.LENGTH_SHORT).show();

                }
                loadingDialog.cancel();
            }
        });
    }

    @Override
    public boolean onOptionsItemSelected(@NonNull MenuItem item) {
        if(item.getItemId() == android.R.id.home)
            finish();
        return super.onOptionsItemSelected(item);
    }
}

ScoreActivity.java

public class ScoreActivity extends AppCompatActivity {

    private TextView score;
    private Button done;

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

        score = findViewById(R.id.score_tv);
        done = findViewById(R.id.score_activity_done);

        String score_str = getIntent().getStringExtra("SCORE");
        final int setNum = getIntent().getIntExtra("SetNum", 1);
        score.setText(score_str);

        done.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                // Here is the issue I am facing
                View view = findViewById(R.id.setNumber);
                view.setBackgroundColor(Color.GREEN);
                Intent mainIntent = new Intent(ScoreActivity.this, MainActivity.class);
                startActivity(mainIntent);
                ScoreActivity.this.finish();
            }
        });


    }
}