So the next step...
I've started using this stuff in the last few years. Water soluble and can be reactivated once dry with a drop of water.
The good thing about that us that mistakes are easy to correct and merging colours is straightforward. The bad thing is that you can easily overwrite something you are happy with and that fingers will lift the paint. More on this later.
I use the above colours and make dilute washes of colours and add to random stones. The washes are mixed in a palette.
I keep randomly mixing and applying colour until I'm happy with it. This is where I'm up to:
Not easy to photograph!
If I were to an a mortar wash now all the stonework colour would be erased. To prevent that you have to apply a coat of varnish. I've got this (again a Hobbycraft sale item) with apparently will protect the colours from fading too...
It does subtly alter the colours / shade though.