Black Rose Immortal vs Mournfang Brown

How close are they?

ΔE 16.7, loose: different enough that you should pick on purpose. Mournfang Brown is a little darker, much more saturated than Black Rose Immortal.

Can I use Mournfang Brown instead of Black Rose Immortal?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Mournfang Brown is a little darker, much more saturated than Black Rose Immortal.

Can I use Black Rose Immortal instead of Mournfang Brown?

The same difference applies the other way round: Black Rose Immortal is a little lighter, much more muted.

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.