Grey Shadow vs Deep Grey

How close are they?

ΔE 9.8, close: similar, with a visible difference side by side. Deep Grey is a little darker, cooler than Grey Shadow.

Can I use Deep Grey instead of Grey Shadow?

Usually, if you are not matching paint already on the model. Close match (ΔE 10). Deep Grey is a little darker, cooler than Grey Shadow.

Can I use Grey Shadow instead of Deep Grey?

The same difference applies the other way round: Grey Shadow is a little lighter, warmer.

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.