Dark Shadow vs Black Grey

How close are they?

ΔE 7.6, close: similar, with a visible difference side by side. Black Grey is a little darker, more muted than Dark Shadow.

Can I use Black Grey instead of Dark Shadow?

Usually, if you are not matching paint already on the model. Close match (ΔE 8). Black Grey is a little darker, more muted than Dark Shadow.

Can I use Dark Shadow instead of Black Grey?

The same difference applies the other way round: Dark Shadow is a little lighter, more saturated.

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.