ΔE 14.0, fair: the same colour family, but not a straight swap. Phthalo Green (Yellow Shade) is a little darker, warmer than Waystone Green. They are different kinds of paint: Citadel Waystone Green is a technical paint or medium 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 14). Phthalo Green (Yellow Shade) is a little darker, warmer than Waystone Green. They are different kinds of paint.
The same difference applies the other way round: Waystone Green is a little lighter, 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 Waystone Green on the colour wheel See Phthalo Green (Yellow Shade) on the colour wheel
All matches: Waystone Green equivalents · Phthalo Green (Yellow Shade) equivalents