Deep Grey vs Darkness Grey

How close are they?

ΔE 8.8, close: similar, with a visible difference side by side. Darkness Grey is a little darker, more muted than Deep Grey.

Can I use Darkness Grey instead of Deep Grey?

Usually, if you are not matching paint already on the model. Close match (ΔE 9). Darkness Grey is a little darker, more muted than Deep Grey.

Can I use Deep Grey instead of Darkness Grey?

The same difference applies the other way round: Deep Grey 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.