Thunderous Blue vs Dark Eternal Night

How close are they?

ΔE 19.1, loose: different enough that you should pick on purpose. Dark Eternal Night is a little darker, much more muted than Thunderous Blue.

Can I use Dark Eternal Night instead of Thunderous Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Dark Eternal Night is a little darker, much more muted than Thunderous Blue.

Can I use Thunderous Blue instead of Dark Eternal Night?

The same difference applies the other way round: Thunderous Blue is a little lighter, much 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.