Dark Sand vs Karak Stone

How close are they?

ΔE 17.6, loose: different enough that you should pick on purpose. Karak Stone is noticeably darker, much more muted than Dark Sand.

Can I use Karak Stone instead of Dark Sand?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Karak Stone is noticeably darker, much more muted than Dark Sand.

Can I use Dark Sand instead of Karak Stone?

The same difference applies the other way round: Dark Sand is noticeably lighter, much 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.