Space Wolves Grey vs Spectral Wolf

How close are they?

ΔE 19.5, loose: different enough that you should pick on purpose. Spectral Wolf is a little lighter, more muted than Space Wolves Grey.

Can I use Spectral Wolf instead of Space Wolves Grey?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Spectral Wolf is a little lighter, more muted than Space Wolves Grey.

Can I use Space Wolves Grey instead of Spectral Wolf?

The same difference applies the other way round: Space Wolves Grey is a little darker, 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.