Guilliman Flesh vs Khaki Drill

How close are they?

ΔE 20.0, loose: different enough that you should pick on purpose. Khaki Drill is noticeably darker, more saturated than Guilliman Flesh.

Can I use Khaki Drill instead of Guilliman Flesh?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Khaki Drill is noticeably darker, more saturated than Guilliman Flesh.

Can I use Guilliman Flesh instead of Khaki Drill?

The same difference applies the other way round: Guilliman Flesh 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.