This is a pretty niche question, I’m sure, but basically what I’d like to know is whether it’s possible for a new app to modify the existing Contacts app in Android. For a (very) simple example, consider an app that allows you to color your girlfriend’s contact slot pink, or highlight all of your family members. Simple things like that, to change the appearance of individual contacts.
Note that I do not want to create a new Contacts app. This is obviously possible and has been done. I simply would like to alter, or perhaps customize, the existing native Android Contacts App.
Nope, unfortunately, if you want to change how the user accesses their contacts, you’ll have to create your own app. There’s no way to build upon Android’s existing default contacts app.