Resplendent Red vs Demon Red

How close are they?

ΔE 21.9, loose: different enough that you should pick on purpose. Demon Red is noticeably lighter, more saturated than Resplendent Red.

Can I use Demon Red instead of Resplendent Red?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 22). Demon Red is noticeably lighter, more saturated than Resplendent Red.

Can I use Resplendent Red instead of Demon Red?

The same difference applies the other way round: Resplendent Red is noticeably darker, 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.