Screaming Skull vs Desert Sand

How close are they?

ΔE 18.9, loose: different enough that you should pick on purpose. Desert Sand is a little lighter, much more saturated than Screaming Skull.

Can I use Desert Sand instead of Screaming Skull?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Desert Sand is a little lighter, much more saturated than Screaming Skull.

Can I use Screaming Skull instead of Desert Sand?

The same difference applies the other way round: Screaming Skull is a little darker, 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.