ΔE 11.5, fair: the same colour family, but not a straight swap. Flat Red is a little lighter, more saturated than Mars Red.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 11). Flat Red is a little lighter, more saturated than Mars Red.
The same difference applies the other way round: Mars Red is a little 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.
See Mars Red on the colour wheel See Flat Red on the colour wheel
All matches: Mars Red equivalents · Flat Red equivalents