XV-88 vs NATO Brown

How close are they?

ΔE 17.8, loose: different enough that you should pick on purpose. NATO Brown is a little darker, much more muted than XV-88.

Can I use NATO Brown instead of XV-88?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). NATO Brown is a little darker, much more muted than XV-88.

Can I use XV-88 instead of NATO Brown?

The same difference applies the other way round: XV-88 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.