ΔE 17.7, loose: different enough that you should pick on purpose. Emperor Red is a little lighter, more muted, cooler than Mephiston Red.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Emperor Red is a little lighter, more muted, cooler than Mephiston Red.
The same difference applies the other way round: Mephiston Red is a little darker, 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 Mephiston Red on the colour wheel See Emperor Red on the colour wheel
All matches: Mephiston Red equivalents · Emperor Red equivalents