RAF Middle Stone vs Mid DAK Sand

How close are they?

ΔE 6.5, close: similar, with a visible difference side by side. Mid DAK Sand is a little lighter than RAF Middle Stone.

Can I use Mid DAK Sand instead of RAF Middle Stone?

Usually, if you are not matching paint already on the model. Close match (ΔE 6). Mid DAK Sand is a little lighter than RAF Middle Stone.

Can I use RAF Middle Stone instead of Mid DAK Sand?

The same difference applies the other way round: RAF Middle Stone is a little darker.

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.