Screaming Skull vs Fang White

How close are they?

ΔE 15.7, loose: different enough that you should pick on purpose. Fang White is noticeably lighter, more muted than Screaming Skull.

Can I use Fang White instead of Screaming Skull?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Fang White is noticeably lighter, more muted than Screaming Skull.

Can I use Screaming Skull instead of Fang White?

The same difference applies the other way round: Screaming Skull is noticeably 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.