Nihilakh Oxide vs Matt Varnish

How close are they?

ΔE 38.2, loose: different enough that you should pick on purpose. Matt Varnish is noticeably lighter, much more muted than Nihilakh Oxide.

Can I use Matt Varnish instead of Nihilakh Oxide?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 38). Matt Varnish is noticeably lighter, much more muted than Nihilakh Oxide.

Can I use Nihilakh Oxide instead of Matt Varnish?

The same difference applies the other way round: Nihilakh Oxide is noticeably darker, 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.