RAF Dark Slate Grey vs Venom Wyrm

How close are they?

ΔE 8.9, close: similar, with a visible difference side by side. Venom Wyrm is a little darker, more saturated than RAF Dark Slate Grey.

Can I use Venom Wyrm instead of RAF Dark Slate Grey?

Usually, if you are not matching paint already on the model. Close match (ΔE 9). Venom Wyrm is a little darker, more saturated than RAF Dark Slate Grey.

Can I use RAF Dark Slate Grey instead of Venom Wyrm?

The same difference applies the other way round: RAF Dark Slate Grey is a little lighter, 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.