ΔE 15.2, loose: different enough that you should pick on purpose. Emerald Green is a little lighter, more saturated, cooler than Niblet Green.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Emerald Green is a little lighter, more saturated, cooler than Niblet Green.
The same difference applies the other way round: Niblet Green is a little darker, more muted, warmer.
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 Emerald Green on the colour wheel
All matches: Niblet Green equivalents · Emerald Green equivalents