Holy White vs Spectral Wolf

How close are they?

ΔE 21.5, loose: different enough that you should pick on purpose. Spectral Wolf is noticeably darker, more saturated than Holy White.

Can I use Spectral Wolf instead of Holy White?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Spectral Wolf is noticeably darker, more saturated than Holy White.

Can I use Holy White instead of Spectral Wolf?

The same difference applies the other way round: Holy White is noticeably 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.