Black Legion vs Dark Brown

How close are they?

ΔE 26.6, loose: different enough that you should pick on purpose. Dark Brown is noticeably lighter, much more saturated than Black Legion.

Can I use Dark Brown instead of Black Legion?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 27). Dark Brown is noticeably lighter, much more saturated than Black Legion.

Can I use Black Legion instead of Dark Brown?

The same difference applies the other way round: Black Legion 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.