ΔE 6.8, close: similar, with a visible difference side by side. Ulthuan Grey is a little darker than Bubble White.
Can I use Ulthuan Grey instead of Bubble White?
Usually, if you are not matching paint already on the model. Close match (ΔE 7). Ulthuan Grey is a little darker than Bubble White.
Can I use Bubble White instead of Ulthuan Grey?
The same difference applies the other way round: Bubble White is a little lighter.
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.