Deep Grey vs Midnight Purple

How close are they?

ΔE 9.9, close: similar, with a visible difference side by side. Midnight Purple is a little darker, warmer than Deep Grey.

Can I use Midnight Purple instead of Deep Grey?

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

Can I use Deep Grey instead of Midnight Purple?

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