ΔE 17.1, loose: different enough that you should pick on purpose. Flat Green is noticeably lighter, more muted, warmer than Sons of Horus Green.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Flat Green is noticeably lighter, more muted, warmer than Sons of Horus Green.
The same difference applies the other way round: Sons of Horus Green is noticeably darker, more saturated, cooler.
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 Sons of Horus Green on the colour wheel See Flat Green on the colour wheel
All matches: Sons of Horus Green equivalents · Flat Green equivalents