Pallid Bone vs Khaki Drill

How close are they?

ΔE 37.6, loose: different enough that you should pick on purpose. Khaki Drill is noticeably darker, more saturated, warmer than Pallid Bone.

Can I use Khaki Drill instead of Pallid Bone?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 38). Khaki Drill is noticeably darker, more saturated, warmer than Pallid Bone.

Can I use Pallid Bone instead of Khaki Drill?

The same difference applies the other way round: Pallid Bone is noticeably lighter, more muted, 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.