ΔE 15.6, loose: different enough that you should pick on purpose. Niblet Green is a little lighter, much more muted than Orc Green.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Niblet Green is a little lighter, much more muted than Orc Green.
The same difference applies the other way round: Orc Green is a little darker, much more saturated.
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 Orc Green on the colour wheel See Niblet Green on the colour wheel
All matches: Orc Green equivalents · Niblet Green equivalents