ΔE 20.8, loose: different enough that you should pick on purpose. Bright Green is a little darker, much more muted, warmer than Niblet Green.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Bright Green is a little darker, much more muted, warmer than Niblet Green.
The same difference applies the other way round: Niblet Green is a little lighter, much more saturated, cooler.
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 Niblet Green on the colour wheel See Bright Green on the colour wheel
All matches: Niblet Green equivalents · Bright Green equivalents