Mournfang Brown vs Bloodstone

How close are they?

ΔE 20.8, loose: different enough that you should pick on purpose. Bloodstone is noticeably lighter, much more saturated than Mournfang Brown.

Can I use Bloodstone instead of Mournfang Brown?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Bloodstone is noticeably lighter, much more saturated than Mournfang Brown.

Can I use Mournfang Brown instead of Bloodstone?

The same difference applies the other way round: Mournfang Brown is noticeably darker, 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.