Mail.ru (the Russian web portal / email service) does photo colorization and restoration very effectively (and for free) in their 9 May project. It's intended for old damaged military photos but I've had good results with various other ones.<p>Engineering write-up: <a href="https://habr.com/ru/company/vk/blog/459696/" rel="nofollow">https://habr.com/ru/company/vk/blog/459696/</a><p>Actual Site: <a href="https://9may.mail.ru/restoration/?lang=en" rel="nofollow">https://9may.mail.ru/restoration/?lang=en</a>
So by app you actually mean webapp? For some reason I thought you meant you created an ios/android app, but I couldn't find any references to this.