ΔE 20.9, loose: different enough that you should pick on purpose. Phthalo Blue (Green Shade) is a little darker, much more saturated, warmer than Sapphire Gem.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Phthalo Blue (Green Shade) is a little darker, much more saturated, warmer than Sapphire Gem.
The same difference applies the other way round: Sapphire Gem is a little lighter, much more muted, 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 Sapphire Gem on the colour wheel See Phthalo Blue (Green Shade) on the colour wheel
All matches: Sapphire Gem equivalents · Phthalo Blue (Green Shade) equivalents