ΔE 14.3, fair: the same colour family, but not a straight swap. Mephiston Red is noticeably darker, cooler than Carmine.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 14). Mephiston Red is noticeably darker, cooler than Carmine.
The same difference applies the other way round: Carmine is noticeably lighter, 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 Carmine on the colour wheel See Mephiston Red on the colour wheel
All matches: Carmine equivalents · Mephiston Red equivalents