Ulthuan Grey vs Grey

How close are they?

ΔE 6.3, close: similar, with a visible difference side by side. Grey is a little darker, more muted than Ulthuan Grey.

Can I use Grey instead of Ulthuan Grey?

Usually, if you are not matching paint already on the model. Close match (ΔE 6). Grey is a little darker, more muted than Ulthuan Grey.

Can I use Ulthuan Grey instead of Grey?

The same difference applies the other way round: Ulthuan Grey is a little lighter, 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.