ΔE 19.5, loose: different enough that you should pick on purpose. Ulthuan Grey is a little lighter, much more muted than Phantom Blue.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Ulthuan Grey is a little lighter, much more muted than Phantom Blue.
The same difference applies the other way round: Phantom Blue is a little darker, much more saturated.
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 Phantom Blue on the colour wheel See Ulthuan Grey on the colour wheel
All matches: Phantom Blue equivalents · Ulthuan Grey equivalents