RAL 7009 Hellgrau vs Middle Stone

How close are they?

ΔE 7.5, close: similar, with a visible difference side by side. Middle Stone is a little lighter, more saturated than RAL 7009 Hellgrau.

Can I use Middle Stone instead of RAL 7009 Hellgrau?

Usually, if you are not matching paint already on the model. Close match (ΔE 8). Middle Stone is a little lighter, more saturated than RAL 7009 Hellgrau.

Can I use RAL 7009 Hellgrau instead of Middle Stone?

The same difference applies the other way round: RAL 7009 Hellgrau is a little darker, 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.