I have done some amateur texturing for 3D shooters and what I very quickly learned was that a good grey texture requires a fair amount of colour.

Here's an example.

That stone texture is full of different colours (the use-many-different-colours principle works with any single-tone texture, really), and I think it would work very well for your greyed-out areas. They'd stand out because they're not as green as the kingdom that you're focusing on, but they'd not look as desaturated.

Hope this makes sense and is of use to you.