ΔE 11.8, fair: the same colour family, but not a straight swap. Phthalo Blue (Green Shade) is a little darker, warmer than Tyran Blue. They are different kinds of paint: Citadel Tyran Blue is a wash/shade and Golden Phthalo Blue (Green 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 12). Phthalo Blue (Green Shade) is a little darker, warmer than Tyran Blue. They are different kinds of paint.
The same difference applies the other way round: Tyran Blue 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 Tyran Blue on the colour wheel See Phthalo Blue (Green Shade) on the colour wheel
All matches: Tyran Blue equivalents · Phthalo Blue (Green Shade) equivalents