Rust vs Khaki Drill

How close are they?

ΔE 28.0, loose: different enough that you should pick on purpose. Khaki Drill is noticeably lighter, much more muted than Rust.

Can I use Khaki Drill instead of Rust?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 28). Khaki Drill is noticeably lighter, much more muted than Rust.

Can I use Rust instead of Khaki Drill?

The same difference applies the other way round: Rust is noticeably darker, 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.