ΔE 13.6, fair: the same colour family, but not a straight swap. Mournfang Brown is a little darker, more saturated, warmer than Umber Brown.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 14). Mournfang Brown is a little darker, more saturated, warmer than Umber Brown.
The same difference applies the other way round: Umber Brown is a little lighter, more muted, 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 Umber Brown on the colour wheel See Mournfang Brown on the colour wheel
All matches: Umber Brown equivalents · Mournfang Brown equivalents