Dark Eternal Night vs Ahriman Blue

How close are they?

ΔE 25.7, loose: different enough that you should pick on purpose. Ahriman Blue is noticeably lighter, much more saturated than Dark Eternal Night.

Can I use Ahriman Blue instead of Dark Eternal Night?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 26). Ahriman Blue is noticeably lighter, much more saturated than Dark Eternal Night.

Can I use Dark Eternal Night instead of Ahriman Blue?

The same difference applies the other way round: Dark Eternal Night is noticeably darker, much more muted.

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.