ΔE 7.3, close: similar, with a visible difference side by side. Sons of Horus Green is a little lighter than Ruinous Spell.
Usually, if you are not matching paint already on the model. Close match (ΔE 7). Sons of Horus Green is a little lighter than Ruinous Spell.
The same difference applies the other way round: Ruinous Spell 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 Ruinous Spell on the colour wheel See Sons of Horus Green on the colour wheel
All matches: Ruinous Spell equivalents · Sons of Horus Green equivalents