Nightshade Purple vs Dark Grey

How close are they?

ΔE 6.3, close: similar, with a visible difference side by side. Dark Grey is more muted than Nightshade Purple.

Can I use Dark Grey instead of Nightshade Purple?

Usually, if you are not matching paint already on the model. Close match (ΔE 6). Dark Grey is more muted than Nightshade Purple.

Can I use Nightshade Purple instead of Dark Grey?

The same difference applies the other way round: Nightshade Purple is 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.