ΔE 12.8, fair: the same colour family, but not a straight swap. Ink Sepia is a little darker, more saturated than Mars Black.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 13). Ink Sepia is a little darker, more saturated than Mars Black.
The same difference applies the other way round: Mars Black is a little lighter, 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 Black on the colour wheel See Ink Sepia on the colour wheel
All matches: Mars Black equivalents · Ink Sepia equivalents