ΔE 10.5, fair: the same colour family, but not a straight swap. NATO Green is a little lighter than Slate Grey.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 11). NATO Green is a little lighter than Slate Grey.
The same difference applies the other way round: Slate Grey is a little darker.
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.
See Slate Grey on the colour wheel See NATO Green on the colour wheel
All matches: Slate Grey equivalents · NATO Green equivalents