I need help with the background

Hey,

So Im kinda stuck with in my first paludarium attempt because I can’t decide what I wanna do with the background.

Either I colour it gray like the slate, or cover it with some kind of substrate.

Or first color it gray, and then cover it with substrate?

And what kind of paint should I use?

I only wanna use it for plants btw

Any tips, recommendations would be greatly appreciated.

Thanks 🙂