Apply custom background color to Android Notification

MyActivity.java

package com.github.aurae.notifications;

import android.app.Notification;
import android.os.Bundle;
import android.support.v4.app.NotificationManagerCompat;
import android.support.v7.app.AppCompatActivity;

public class MyActivity extends AppCompatActivity {

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

		Notification.Builder b = new Notification.Builder(this)
				.setSmallIcon(android.R.drawable.btn_star)
				.setContentTitle("title")
				.setContentText("subtitle");
		
		Notification n = NotificationExtras.buildWithBackgroundColor(this, b, 0xffff0000);
		// Notification n = NotificationExtras.buildWithBackgroundResource(this, b, R.drawable.bg_notification);
		NotificationManagerCompat.from(this).notify(1, n);

	}
}

NotificationExtras.java

package com.github.aurae.notifications;

import android.app.Notification;
import android.content.Context;
import android.graphics.Color;
import android.os.Build;
import android.support.annotation.ColorInt;
import android.support.annotation.DrawableRes;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.RemoteViews;
import android.widget.TextView;

import java.lang.reflect.Method;

public final class NotificationExtras {

	// Method reference to Notification.Builder#makeContentView
	private static final Method MAKE_CONTENT_VIEW_METHOD;

	static {
		Method m = null;
		try {
			m = Notification.Builder.class.getDeclaredMethod("makeContentView");
			m.setAccessible(true);
		} catch (NoSuchMethodException e) {
			e.printStackTrace();
		}
		MAKE_CONTENT_VIEW_METHOD = m;
	}

	private NotificationExtras() {
		// no instance
	}

	/* Public */

	public static Notification buildWithBackgroundResource(Context context, Notification.Builder builder, @DrawableRes int res) {
		if (MAKE_CONTENT_VIEW_METHOD == null) return buildNotification(builder);
		RemoteViews remoteViews = obtainRemoteViews(builder);
		Notification notification = buildNotification(builder);

		// Find the root of the content view and apply the background to it
		if (remoteViews != null) {
			View v = LayoutInflater.from(context).inflate(remoteViews.getLayoutId(), null);
			remoteViews.setInt(v.getId(), "setBackgroundResource", res);
		}

		return notification;
	}

	public static Notification buildWithBackgroundColor(Context context, Notification.Builder builder, @ColorInt int color) {
		if (MAKE_CONTENT_VIEW_METHOD == null) return buildNotification(builder);
		RemoteViews remoteViews = obtainRemoteViews(builder);
		Notification notification = buildNotification(builder);

		// Find the root of the content view and apply the color to it
		if (remoteViews != null) {
			View v = LayoutInflater.from(context).inflate(remoteViews.getLayoutId(), null);
			remoteViews.setInt(v.getId(), "setBackgroundColor", color);

			// Calculate a contrasting text color to ensure readability, and apply it to all TextViews within the notification layout
			boolean useDarkText = (Color.red(color) * 0.299 + Color.green(color) * 0.587 + Color.blue(color) * 0.114 > 186);
			int textColor = useDarkText ? 0xff000000 : 0xffffffff;
			applyTextColorToRemoteViews(remoteViews, v, textColor);
		}

		return notification;
	}

	/* Private */

	private static RemoteViews obtainRemoteViews(Notification.Builder builder) {
		try {
			// Explicitly force creation of the content view and re-assign it to the notification
			RemoteViews remoteViews = (RemoteViews) MAKE_CONTENT_VIEW_METHOD.invoke(builder);
			builder.setContent(remoteViews);
			return remoteViews;

		} catch (Throwable ignored) {
			return null;
		}
	}

	private static Notification buildNotification(Notification.Builder builder) {
		if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.JELLY_BEAN) {
			return builder.build();
		} else {
			return builder.getNotification();
		}
	}

	private static void applyTextColorToRemoteViews(RemoteViews remoteViews, View view, int color) {
		if (view instanceof ViewGroup) {
			ViewGroup vg = (ViewGroup) view;
			for (int i = 0, count = vg.getChildCount(); i < count; i++) {
				applyTextColorToRemoteViews(remoteViews, vg.getChildAt(i), color);
			}
		} else if (view instanceof TextView) {
			remoteViews.setTextColor(view.getId(), color);
		}
	}
}

Not working....

You have not even set channel for notification

Notification channels did not exist when I wrote this. I recommend not using this for anything in 2022.