ΔE 14.0, fair: the same colour family, but not a straight swap. Grass Green is a little lighter, more saturated, warmer than Ruinous Spell.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 14). Grass Green is a little lighter, more saturated, warmer than Ruinous Spell.
The same difference applies the other way round: Ruinous Spell is a little darker, 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 Ruinous Spell on the colour wheel See Grass Green on the colour wheel
All matches: Ruinous Spell equivalents · Grass Green equivalents