ΔE 19.9, loose: different enough that you should pick on purpose. Flesh Wash is much more saturated than Soulblight Grey.
Can I use Flesh Wash instead of Soulblight Grey?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Flesh Wash is much more saturated than Soulblight Grey.
Can I use Soulblight Grey instead of Flesh 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.