ΔE 11.8, fair: the same colour family, but not a straight swap. Ulthuan Grey is a little lighter than Pale Blue.
Can I use Ulthuan Grey instead of Pale Blue?
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 12). Ulthuan Grey is a little lighter than Pale Blue.
Can I use Pale Blue instead of Ulthuan Grey?
The same difference applies the other way round: Pale Blue is a little darker.
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.