Bony Matter vs Khaki Drill

How close are they?

ΔE 15.6, loose: different enough that you should pick on purpose. Khaki Drill is noticeably darker, more saturated than Bony Matter.

Can I use Khaki Drill instead of Bony Matter?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Khaki Drill is noticeably darker, more saturated than Bony Matter.

Can I use Bony Matter instead of Khaki Drill?

The same difference applies the other way round: Bony Matter 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.