ΔE 7.4, close: similar, with a visible difference side by side. Niblet Green is a little darker, warmer than Spectral Green.
Usually, if you are not matching paint already on the model. Close match (ΔE 7). Niblet Green is a little darker, warmer than Spectral Green.
The same difference applies the other way round: Spectral Green is a little lighter, 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 Spectral Green on the colour wheel See Niblet Green on the colour wheel
All matches: Spectral Green equivalents · Niblet Green equivalents