ΔE 12.0, fair: the same colour family, but not a straight swap. Phthalo Green (Blue Shade) is noticeably darker than Hexwraith Flame. They are different kinds of paint: Citadel Hexwraith Flame is a one-coat contrast-style paint and Golden Phthalo Green (Blue Shade) is an opaque base/layer 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 12). Phthalo Green (Blue Shade) is noticeably darker than Hexwraith Flame. They are different kinds of paint.
The same difference applies the other way round: Hexwraith Flame is noticeably lighter.
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 Hexwraith Flame on the colour wheel See Phthalo Green (Blue Shade) on the colour wheel
All matches: Hexwraith Flame equivalents · Phthalo Green (Blue Shade) equivalents