ΔE 10.6, fair: the same colour family, but not a straight swap. Brown RLM26 is a little lighter, cooler than Mournfang Brown.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 11). Brown RLM26 is a little lighter, cooler than Mournfang Brown.
The same difference applies the other way round: Mournfang Brown is a little darker, 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 Mournfang Brown on the colour wheel See Brown RLM26 on the colour wheel
All matches: Mournfang Brown equivalents · Brown RLM26 equivalents