ΔE 15.0, loose: different enough that you should pick on purpose. Phthalo Green (Blue Shade) is a little lighter, much more saturated than Guardian Green.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Phthalo Green (Blue Shade) is a little lighter, much more saturated than Guardian Green.
The same difference applies the other way round: Guardian Green is a little darker, much more muted.
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 Guardian Green on the colour wheel See Phthalo Green (Blue Shade) on the colour wheel
All matches: Guardian Green equivalents · Phthalo Green (Blue Shade) equivalents