Name of the app that changes the lighting from reg. to dark in order to make out faint lines!

Ol

Hi ladies! I’ve seen a bunch of test pictures on here and I’m wondering what the app is that people use to change the lighting to almost like an x-ray light (or that’s how I think of it lol). In order to see faint lines that may be hard to see with the naked eye. If there’s no app, does anyone know how to do it?