ΔE 14.6, fair: the same colour family, but not a straight swap. Battle of Marathon is a little darker, much more muted, warmer than Ruddy Umber.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 15). Battle of Marathon is a little darker, much more muted, warmer than Ruddy Umber.
The same difference applies the other way round: Ruddy Umber is a little lighter, much more saturated, 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 Ruddy Umber on the colour wheel See Battle of Marathon on the colour wheel
All matches: Ruddy Umber equivalents · Battle of Marathon equivalents