Incubi Darkness vs Bluegrey Dusk

How close are they?

ΔE 16.3, loose: different enough that you should pick on purpose. Bluegrey Dusk is a little darker, warmer than Incubi Darkness.

Can I use Bluegrey Dusk instead of Incubi Darkness?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Bluegrey Dusk is a little darker, warmer than Incubi Darkness.

Can I use Incubi Darkness instead of Bluegrey Dusk?

The same difference applies the other way round: Incubi Darkness 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.