ΔE 14.1, fair: the same colour family, but not a straight swap. Flat Green is a little darker, more saturated, warmer than Highland Moss.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 14). Flat Green is a little darker, more saturated, warmer than Highland Moss.
The same difference applies the other way round: Highland Moss is a little lighter, more muted, 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 Highland Moss on the colour wheel See Flat Green on the colour wheel
All matches: Highland Moss equivalents · Flat Green equivalents