Ruinous Spell vs Green Morkshroom

How close are they?

ΔE 7.5, close: similar, with a visible difference side by side. Green Morkshroom is a little darker, more muted than Ruinous Spell.

Can I use Green Morkshroom instead of Ruinous Spell?

Usually, if you are not matching paint already on the model. Close match (ΔE 8). Green Morkshroom is a little darker, more muted than Ruinous Spell.

Can I use Ruinous Spell instead of Green Morkshroom?

The same difference applies the other way round: Ruinous Spell is a little lighter, more saturated.

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.