ΔE 24.5, loose: different enough that you should pick on purpose. Scarlett Red is noticeably lighter, much more saturated, warmer than Mephiston Red.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 25). Scarlett Red is noticeably lighter, much more saturated, warmer than Mephiston Red.
The same difference applies the other way round: Mephiston Red is noticeably darker, much more muted, cooler.
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 Mephiston Red on the colour wheel See Scarlett Red on the colour wheel
All matches: Mephiston Red equivalents · Scarlett Red equivalents