ΔE 12.9, fair: the same colour family, but not a straight swap. Brown is noticeably darker, cooler than Ruddy Umber.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 13). Brown is noticeably darker, cooler than Ruddy Umber.
The same difference applies the other way round: Ruddy Umber is noticeably lighter, 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 Ruddy Umber on the colour wheel See Brown on the colour wheel
All matches: Ruddy Umber equivalents · Brown equivalents