Navy Blue vs Gray Violet (Luftwaffe)

How close are they?

ΔE 15.9, loose: different enough that you should pick on purpose. Gray Violet (Luftwaffe) is noticeably lighter, more muted than Navy Blue.

Can I use Gray Violet (Luftwaffe) instead of Navy Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Gray Violet (Luftwaffe) is noticeably lighter, more muted than Navy Blue.

Can I use Navy Blue instead of Gray Violet (Luftwaffe)?

The same difference applies the other way round: Navy Blue is noticeably darker, 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.