Archangel Red vs Demon Red

How close are they?

ΔE 19.0, loose: different enough that you should pick on purpose. Demon Red is a little lighter, much more muted than Archangel Red.

Can I use Demon Red instead of Archangel Red?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Demon Red is a little lighter, much more muted than Archangel Red.

Can I use Archangel Red instead of Demon Red?

The same difference applies the other way round: Archangel Red is a little darker, 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.