ΔE 17.0, loose: different enough that you should pick on purpose. Light Phthalo Blue is a little lighter, much more saturated than Ulthuan Grey. They are different kinds of paint: Citadel Ulthuan Grey is an opaque base/layer paint and Golden Light Phthalo Blue is an ink, glaze or transparent paint, so they won’t behave the same on the model even where the colour is close.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Light Phthalo Blue is a little lighter, much more saturated than Ulthuan Grey. They are different kinds of paint.
The same difference applies the other way round: Ulthuan Grey 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 Ulthuan Grey on the colour wheel See Light Phthalo Blue on the colour wheel
All matches: Ulthuan Grey equivalents · Light Phthalo Blue equivalents