ΔE 16.1, loose: different enough that you should pick on purpose. Russ. AF Light Blue is more saturated than Ulthuan Grey.
Can I use Russ. AF Light Blue instead of Ulthuan Grey?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Russ. AF Light Blue is more saturated than Ulthuan Grey.
Can I use Ulthuan Grey instead of Russ. AF Light Blue?
The same difference applies the other way round: Ulthuan Grey is 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.