ΔE 22.8, loose: different enough that you should pick on purpose. Phthalo Green (Blue Shade) is noticeably darker, more muted, cooler than Niblet Green.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 23). Phthalo Green (Blue Shade) is noticeably darker, more muted, cooler than Niblet Green.
The same difference applies the other way round: Niblet Green is noticeably lighter, more saturated, 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 Phthalo Green (Blue Shade) on the colour wheel
All matches: Niblet Green equivalents · Phthalo Green (Blue Shade) equivalents