Matt White Primer vs Nihilakh Oxide

How close are they?

ΔE 44.4, loose: different enough that you should pick on purpose. Nihilakh Oxide is noticeably darker, much more saturated than Matt White Primer.

Can I use Nihilakh Oxide instead of Matt White Primer?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 44). Nihilakh Oxide is noticeably darker, much more saturated than Matt White Primer.

Can I use Matt White Primer instead of Nihilakh Oxide?

The same difference applies the other way round: Matt White Primer is noticeably lighter, much 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.