ΔE 23.2, loose: different enough that you should pick on purpose. Flat Red is noticeably darker, much more muted, cooler than Demon Red.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 23). Flat Red is noticeably darker, much more muted, cooler than Demon Red.
The same difference applies the other way round: Demon Red is noticeably lighter, much more saturated, warmer.
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.
See Demon Red on the colour wheel See Flat Red on the colour wheel
All matches: Demon Red equivalents · Flat Red equivalents