ΔE 6.6, close: similar, with a visible difference side by side. The Darkness is more muted than Russian Blue Lights.
Usually, if you are not matching paint already on the model. Close match (ΔE 7). The Darkness is more muted than Russian Blue Lights.
The same difference applies the other way round: Russian Blue Lights 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.
See Russian Blue Lights on the colour wheel See The Darkness on the colour wheel
All matches: Russian Blue Lights equivalents · The Darkness equivalents