ΔE 23.9, loose: different enough that you should pick on purpose. Grey Blue is a little lighter, much more muted than Ionic Blue.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 24). Grey Blue is a little lighter, much more muted than Ionic Blue.
The same difference applies the other way round: Ionic 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 Ionic Blue on the colour wheel See Grey Blue on the colour wheel
All matches: Ionic Blue equivalents · Grey Blue equivalents