ΔE 13.2, fair: the same colour family, but not a straight swap. Niblet Green is noticeably darker than Ferocious Green.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 13). Niblet Green is noticeably darker than Ferocious Green.
The same difference applies the other way round: Ferocious Green is noticeably lighter.
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 Ferocious Green on the colour wheel See Niblet Green on the colour wheel
All matches: Ferocious Green equivalents · Niblet Green equivalents