Ulthuan Grey vs Tooth and Nails

How close are they?

ΔE 22.9, loose: different enough that you should pick on purpose. Tooth and Nails is a little lighter, more saturated than Ulthuan Grey.

Can I use Tooth and Nails instead of Ulthuan Grey?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 23). Tooth and Nails is a little lighter, more saturated than Ulthuan Grey.

Can I use Ulthuan Grey instead of Tooth and Nails?

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