ΔE 14.0, fair: the same colour family, but not a straight swap. Dull Red is noticeably lighter than Mournfang Brown.
Can I use Dull Red instead of Mournfang Brown?
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 14). Dull Red is noticeably lighter than Mournfang Brown.
Can I use Mournfang Brown instead of Dull Red?
The same difference applies the other way round: Mournfang Brown is noticeably darker.
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.