Sand Grey vs Middle Stone

How close are they?

ΔE 9.3, close: similar, with a visible difference side by side. Middle Stone is more muted, warmer than Sand Grey.

Can I use Middle Stone instead of Sand Grey?

Usually, if you are not matching paint already on the model. Close match (ΔE 9). Middle Stone is more muted, warmer than Sand Grey.

Can I use Sand Grey instead of Middle Stone?

The same difference applies the other way round: Sand Grey is more saturated, cooler.

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.