Violent Vermillion vs Apocalypse Sky

How close are they?

ΔE 15.2, loose: different enough that you should pick on purpose. Apocalypse Sky is a little darker, much more saturated than Violent Vermillion.

Can I use Apocalypse Sky instead of Violent Vermillion?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Apocalypse Sky is a little darker, much more saturated than Violent Vermillion.

Can I use Violent Vermillion instead of Apocalypse Sky?

The same difference applies the other way round: Violent Vermillion is a little lighter, 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.