ΔE 14.6, fair: the same colour family, but not a straight swap. Phthalo Blue (Green Shade) is a little lighter, much more saturated than Ultramarines Blue. They are different kinds of paint: Citadel Ultramarines Blue is a one-coat contrast-style paint and Golden Phthalo Blue (Green 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 15). Phthalo Blue (Green Shade) is a little lighter, much more saturated than Ultramarines Blue. They are different kinds of paint.
The same difference applies the other way round: Ultramarines Blue 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 Ultramarines Blue on the colour wheel See Phthalo Blue (Green Shade) on the colour wheel
All matches: Ultramarines Blue equivalents · Phthalo Blue (Green Shade) equivalents