Ancient Stone vs Bone Khaki

How close are they?

ΔE 18.3, loose: different enough that you should pick on purpose. Bone Khaki is noticeably darker, warmer than Ancient Stone.

Can I use Bone Khaki instead of Ancient Stone?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Bone Khaki is noticeably darker, warmer than Ancient Stone.

Can I use Ancient Stone instead of Bone Khaki?

The same difference applies the other way round: Ancient Stone is noticeably lighter, cooler.

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.