Apocalypse Sky vs Sanctuary Red

How close are they?

ΔE 19.4, loose: different enough that you should pick on purpose. Sanctuary Red is a little darker, much more saturated than Apocalypse Sky.

Can I use Sanctuary Red instead of Apocalypse Sky?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Sanctuary Red is a little darker, much more saturated than Apocalypse Sky.

Can I use Apocalypse Sky instead of Sanctuary Red?

The same difference applies the other way round: Apocalypse Sky 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.