Android apps interaction

I'm investigating if I can mak开发者_开发百科e an app that runs in background but is able to interact with others by "enriching" texts adding relevant information. For Instance, interact with an existent twitter app colouring words considered to be relevant. Thanks in advacen for any help

No, you really cannot without the application(s) in question directly invoking or intentionally calling upon your application for the desired functionality.