ΔE 11.1, fair: the same colour family, but not a straight swap. Dark Umber is noticeably darker, warmer than Rifle Brown.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 11). Dark Umber is noticeably darker, warmer than Rifle Brown.
The same difference applies the other way round: Rifle Brown is noticeably lighter, 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 Rifle Brown on the colour wheel See Dark Umber on the colour wheel
All matches: Rifle Brown equivalents · Dark Umber equivalents