Stone is not the same colour across a single slab.

Mosses grow in shade

Rocks are often different colours (slab vs rocks)

Rock's can rust and this is a great way to add points of interest in the base.

I'm far from an expert and most of this comes from the teachings of Seth Amsden who manages the FLGS I used to hit up in the US and can talk your ear off about basing and realism. His Crystal Brush entry for 2018 really highlights his skill in this regard.

- Raggy, signing out