ΔE 10.6, fair: the same colour family, but not a straight swap. Phthalo Green (Yellow Shade) is a little lighter, warmer than Guardian Green. They are different kinds of paint: Army Painter Guardian Green is an opaque base/layer paint and Golden Phthalo Green (Yellow Shade) is an ink, glaze or transparent paint, so they won’t behave the same on the model even where the colour is close.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 11). Phthalo Green (Yellow Shade) is a little lighter, warmer than Guardian Green. They are different kinds of paint.
The same difference applies the other way round: Guardian Green is a little darker, 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 Guardian Green on the colour wheel See Phthalo Green (Yellow Shade) on the colour wheel
All matches: Guardian Green equivalents · Phthalo Green (Yellow Shade) equivalents