ΔE 12.9, fair: the same colour family, but not a straight swap. Greasy Black is noticeably lighter than Black Legion.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 13). Greasy Black is noticeably lighter than Black Legion.
The same difference applies the other way round: Black Legion 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.
See Black Legion on the colour wheel See Greasy Black on the colour wheel
All matches: Black Legion equivalents · Greasy Black equivalents