ΔE 19.3, loose: different enough that you should pick on purpose. Necrosis Green Wash is much more saturated than Soulblight Grey.
Can I use Necrosis Green Wash instead of Soulblight Grey?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Necrosis Green Wash is much more saturated than Soulblight Grey.
Can I use Soulblight Grey instead of Necrosis Green Wash?
The same difference applies the other way round: Soulblight Grey is much more muted.
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.