Verdigris vs Ulthuan Grey

How close are they?

ΔE 29.0, loose: different enough that you should pick on purpose. Ulthuan Grey is a little darker, much more muted than Verdigris.

Can I use Ulthuan Grey instead of Verdigris?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 29). Ulthuan Grey is a little darker, much more muted than Verdigris.

Can I use Verdigris instead of Ulthuan Grey?

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