ΔE 15.0, loose: different enough that you should pick on purpose. Sons of Horus Green is a little darker, more saturated, cooler than Field Grey.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Sons of Horus Green is a little darker, more saturated, cooler than Field Grey.
The same difference applies the other way round: Field Grey is a little lighter, more muted, warmer.
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 Field Grey on the colour wheel See Sons of Horus Green on the colour wheel
All matches: Field Grey equivalents · Sons of Horus Green equivalents