ΔE 16.9, loose: different enough that you should pick on purpose. Emerald is a little darker, much more muted, cooler than Phthalo Green (Blue Shade).
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Emerald is a little darker, much more muted, cooler than Phthalo Green (Blue Shade).
The same difference applies the other way round: Phthalo Green (Blue Shade) is a little lighter, much 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 Phthalo Green (Blue Shade) on the colour wheel See Emerald on the colour wheel
All matches: Phthalo Green (Blue Shade) equivalents · Emerald equivalents