Karak Stone vs Middle Stone

How close are they?

ΔE 9.7, close: similar, with a visible difference side by side. Middle Stone is a little darker, more saturated than Karak Stone.

Can I use Middle Stone instead of Karak Stone?

Usually, if you are not matching paint already on the model. Close match (ΔE 10). Middle Stone is a little darker, more saturated than Karak Stone.

Can I use Karak Stone instead of Middle Stone?

The same difference applies the other way round: Karak Stone is a little lighter, more muted.

ΔE is the colour difference between our two swatch estimates: under about 6 is close, over about 15 is loose. Real paints also differ in finish, coverage and how they thin, so test before you commit.